People are scared,” Donald Trump said recently, and he was not wrong.

Fear is in the air, and fear is surging. Americans are more afraid today than they have been in a long time: Polls show majorities of Americans worried about being victims of terrorism and crime, numbers that have surged over the past year to highs not seen for more than a decade. Every week seems to bring a new large- or small-scale terrorist attack, at home or abroad. Mass shootings form a constant drumbeat. Protests have shut down large cities repeatedly, and some have turned violent. Overall crime rates may be down, but a sense of disorder is constant.

Fear pervades Americans’ lives—and American politics. Trump is a master of fear, invoking it in concrete and abstract ways, summoning and validating it. More than most politicians, he grasps and channels the fear coursing through the electorate. And if Trump still stands a chance to win in November, fear could be the key.

Fear and anger are often cited in tandem as the sources of Trump’s particular political appeal, so frequently paired that they become a refrain: fear-and-anger, anger-and-fear. But fear is not the same as anger; it is a unique political force. Its ebbs and flows through American political history have pulled on elections, reordering and destabilizing the electoral landscape.

This week, Trump delivered a speech on immigration that depicted outsiders as a frightening threat. “Countless innocent American lives have been stolen because our politicians have failed in their duty to secure our borders,” he said. His acceptance speech at the Republican National Convention similarly made clear the extent to which his message revolves around fear. “The attacks on our police, and the terrorism in our cities, threaten our very way of life,” Trump thundered. “Any politician who does not grasp this danger is not fit to lead our country. Americans watching this address tonight have seen the recent images of violence in our streets and the chaos in our communities. Many have witnessed this violence personally; some have even been its victims.”

Notes of uplift were few and far between in the convention speech, and commentators were duly shocked by its dark tone. (The conservative writer Reed Galen called Trump’s convention “a fear-fueled acid trip.”) Trump summons fear in the conventional way, by describing in concrete terms the threats Americans face. But he also, in a more unusual maneuver, summons fear in the abstract: There’s something going on, folks.

The critics who accuse Trump of cheap fear-mongering may be failing to recognize that the fear percolating in society is real, and somewhat justified; politicians who fail to validate it risk falling out of step with the zeitgeist. They are likely right, however, that ratcheting up fear helps Trump. This is the way fear works, according to social scientists: It makes people hold more tightly to what they have and regard the unfamiliar more warily. It makes them want to be protected. The fear reaction is a universal one to which everyone is susceptible. It might even be the only way Trump could win.

If the normal categories hold in this election—the patterns of turnout, the states in play, the partisan and demographic divides—it is almost impossible for Trump to prevail. The current polls show him losing in just such a predictable way, dogged by his offenses against various groups. But fear, history shows, has the power to jar voters out of their normal categories.

Trump paints a fearful picture, and events validate his vision. This is what happened in the Republican primary: When back-to-back terror attacks hit Paris in November and San Bernardino in December, he pointed to them as proof that his warnings about Muslims were justified, and voters flocked to him, boosting and solidifying his polling lead in the final stretch before primary voting began. Trump’s standing in the polls rose about 7 percentage points in the aftermath of the attacks, buoying him to the level it would take to win primary contests.

Now, Trump is again leaning into voters’ unease. So far, it doesn’t seem to be working, but events could yet change the equation; this is why many pundits and political scientists believe a large-scale terrorist attack on the eve of the election would redound to Trump’s electoral favor—by validating the fearful vision he has espoused.

Trump supporters, recent polling has shown, are disproportionately fearful. They fear crime and terror far more than other Americans; they are also disproportionately wary of foreign influence and social change. (They are not, however, any more likely than other Americans to express economic anxiety.)

“I used to fly a lot, but now I don’t get on an airplane unless I have to,” Pat Garverick, a retired tech worker, told me at a recent Trump rally in Northern Virginia. “There’s that little voice in the back of your head that says, ‘Is this safe?’ I try to stay away from crowds. There are so many people trying to hurt us or stir up violence.”

Not all the Trump supporters I have asked in recent months say they feel afraid. One woman told me, “I’m not scared; I’m pissed off.” Others cited less immediate fears: They say they are afraid for their country or their children’s future. But many cited a visceral sense of insecurity. “I am terrified,” confided Jonnianne Ridzelski, who I met at a Trump rally in Alabama in April. She had, she said, been making preparations for disaster, including stocking up on canned food.

What, exactly, was she afraid of? She couldn’t say, and that was perhaps the most frightening thing of all. “I don’t know what’s going to happen,” she said.

While anger makes people aggressive, prone to lash out, fear makes them cower from the unfamiliar and seek refuge and comfort. Trump channels people’s anger, but he salves their fear with promises of protection, toughness, strength. It is a feedback loop: He stirs up people’s latent fears, then offers himself as the only solution.

Frightened people come to Trump for reassurance, and he promises to make them feel safe. “I’m scared,” a 12-year-old girl told the candidate at a rally in North Carolina in December. “What are you going to do to protect this country?”

“You know what, darling?” Trump replied. “You’re not going to be scared anymore. They’re going to be scared.”

To the seasoned political practitioner, fear is a handy tool. “Fear is easy,” Rick Wilson, a Florida-based Republican ad maker, told me recently. “Fear is the simplest emotion to tweak in a campaign ad. You associate your opponent with terror, with fear, with crime, with causing pain and uncertainty.”

Wilson has plenty of experience. In 2002, he made a commercial that criticized Democratic Senator Max Cleland, who had lost three limbs in Vietnam, while showing images of Saddam Hussein and Osama bin Laden. In 2008, Wilson made ads attacking Barack Obama by showing the incendiary statements of his former pastor, Jeremiah Wright. “I wanted to scare the living shit out of white people in Pennsylvania and Ohio,” Wilson said. “Today, they would all be Trump voters, I’m sure.”

Fear-based appeals hit people on a primitive level, Wilson said. “When people are under stress, the hind brain takes over,” he said. Trump, Wilson believes, has expertly manipulated many people’s latent fear of the other. “Fear of Mexicans, fear of the Chinese, fear of African Americans—Donald Trump has very deliberately stoked it and inflamed it and made it a centerpiece of his campaign,” he told me.

A majority of Americans now worry that they or their families will be victims of terrorism, up from a third less than two years ago, according to a survey by the Public Religion Research Institute. Nearly two-thirds worry about being victims of violent crime. Another poll, by Gallup, found that concern about crime and violence is at its highest level in 15 years.

Trump supporters are more concerned than most. According to data provided by the Public Religion Research Institute, 65 percent of Trump supporters feared being victims of terrorism, versus 51 percent of all Americans. Three-fourths of Trump supporters feared being victims of crime, versus 63 percent overall. Trump supporters also disproportionately feared foreign influence: 83 percent said the American way of life needed to be protected from it, versus 55 percent overall. Two-thirds of Trump supporters also worried that they or a family member would become unemployed, but this was not much different than the 63 percent of non-Trump supporters who had the same concern. Economic anxiety, while widespread in America today, is not a distinguishing characteristic of Trump supporters; other anxieties are.

Trump’s audience of conservative-leaning voters may be particularly susceptible to fear-based appeals. Researchers have found that those who are more sensitive to threats and more wary of the unfamiliar tend to be more politically conservative. “The common basis for all the various components of the conservative attitude syndrome is a generalized susceptibility to experiencing threat or anxiety in the face of uncertainty,” the British psychologist G.D. Wilson wrote in his 1973 book, The Psychology of Conservatism. In other words, an innate fear of uncertainty tends to correlate to people’s level of conservatism.

Subsequent experiments have confirmed this idea. In a 2003 paper reviewing five decades of research across 12 different countries, the psychologist John Jost and his collaborators found “the psychological management of uncertainty and fear” to be strongly and consistently correlated with politically conservative attitudes. (This “fear of threat,” however, is not the same as anxiety in the sense of neuroticism, which correlates strongly with liberal political attitudes.)

In study after study, the characteristic most predictive of a person’s political leanings is his or her tolerance for ambiguity. “The more intolerant of ambiguity you are—the more you seek control over your surroundings, certainty, clear answers to things—the more you tend toward conservative preferences,” Anat Shenker, a liberal communications consultant and cognitive linguistics researcher, told me.

But it is not only conservatives who are susceptible to fear. Almost all of us exist somewhere on the continuum between the extremes of “totally averse to the unfamiliar” and “totally enthusiastic about the unknown.” Experiments find that everyone’s political views become more conservative when they are provoked to become more fearful. In one study, liberal subjects who had just been confronted with a threat immediately reported more conservative views on abortion, capital punishment, and gay rights.

If fear is strong enough, it can accomplish something exceedingly rare: It can override people’s preexisting partisan commitments. This happened in the wake of the September 11 attacks: Political scientists say Republicans’ success in the 2002 and 2004 elections can be largely attributed to Americans’ increased fear of terrorism. “There is evidence from 2002 and 2004 that people’s concern about terror was a very good predictor of their voting habits, even apart from partisanship,” Shana Gadarian, a political scientist at Syracuse University and the author of The Politics of Threat: How Terrorism News Shapes Foreign Policy Attitudes, told me. (Democrats, Gadarian notes, also use fear to push their agenda on issues with which they’re associated, like climate change and health care.)

Shenker makes the case that the world is changing these days more quickly than any of us are inherently equipped to handle. “The modern condition of life is pretty much an assault on our brains,” she told me. “We’re experiencing change and ambiguity at a rate unprecedented in human history. Think about how long it took to get from the agricultural revolution to the industrial revolution. And now all of a sudden the climate is changing, women are becoming men, I’m talking to you on a little sliver of plastic and metal. We have change in every dimension faster than our brains have evolved to deal with it.” In studying Trump voters on behalf of MoveOn.org, Shenker found that they responded strongly to the idea that he would bring order and control to a chaotic world.

Gadarian, the political scientist, said, “When people feel anxious, they want to be protected.” Trump’s policies, she pointed out, are a literal answer to this desire: protectionist economics; a wall that physically protects the country from outsiders. “How do you overcome the threat of terror, of crime, of immigration? You say, ‘We will protect the country by building a wall.’”

Here is a case study in the power of fear in politics. Immigration reform has seemed ripe for bipartisan compromise ever since George W. Bush tried to pass it in during his second term. Majorities of voters consistently say they support allowing undocumented immigrants to become citizens and oppose mass deportation. Yet the policy has been derailed by intense, concentrated, visceral opposition. Meanwhile, the reaction to mass migration has upended the politics of virtually every European nation, from Brexit to France to Scandinavia.

Frank Sharry, a proponent of immigration reform who heads the group America’s Voice, has worked on the issue since the 1980s, but the rise of Trump forced him to revise his understanding. What had always seemed to him like a policy dispute now strikes him as something more profound and primal, he told me.

“Ten years ago, when [John] McCain and [Ted] Kennedy were working together on comprehensive immigration reform and George W. Bush supported it, I really thought this was a rational policy disagreement that was headed toward a logical compromise,” Sharry told me recently. “Now, I see it as deeply cultural. It’s racially charged, it’s tribalism, it’s us-vs.-them. It’s a referendum on the face of globalization, on a moment of demographic and cultural change.”

There are legitimate policy arguments against increasing immigration or legalizing the undocumented, but Sharry came to believe that they were not the drivers of opposition to the issue. Once you see fear as an axis, it resonates across any number of political debates. The fearful mind sees immigrants as an invasion force, refugees as terrorists, rising crime as a threat to one’s family, drugs as a threat to one’s children, and social change as a threat to one’s way of life. Almost everyone is somewhat susceptible to fear’s appeal; those naturally inclined to be conservative somewhat more so. But it takes a particular type of politician to push the buttons in human nature that activate these fears.

“Some people’s sense of who we are as a country is threatened to the core,” Sharry said. “Trump speaks to our id, something latent in all of us to different degrees. This is not a political campaign. It’s an identity campaign.”

Fear as a political force comes and goes, ebbing and flowing in American history. Politicians have always played to it: Lyndon B. Johnson’s “Daisy” ad envisioned a Barry Goldwater presidency leading to nuclear war; Richard Nixon emphasized “law and order” as a counterweight to the riots of 1968; fears of crime—with racial overtones—produced the “Willie Horton” ad in 1988 and the lock-’em-up mania of the 1990s. Ronald Reagan’s 1984 “Bear” ad—“There is a bear in the woods … Isn’t it smart to be as strong as the bear?”—was echoed by a 2004 George W. Bush ad featuring prowling wolves.

“Fear is present constantly in American politics,” David Bennett, a historian and the author of The Party of Fear: The American Far Right From Nativism to the Militia Movement, told me. The most persistent fear in American life, he said, has been fear of outsiders.

“People need to displace and project their anxieties, their concerns about their own lives and the lives of people they care about, onto some other,” he said. Often they are susceptible to politicians who tell them that “the wrong kinds of people are responsible for threatening them or their loved ones.”

From colonial times to the early 19th century, the pervasive, virulent fear was of Catholics, who were seen as inferior, unassimilable, and in thrall to a foreign dictator (the Pope). The mass immigration of Irish Catholics in the 1830s and 1840s ratcheted up the panic and convulsed American politics, with the Whig Party collapsing and the anti-Catholic nativist Know-Nothing Party briefly becoming America’s second-largest political party.

After the Civil War, a new influx of Italians, Slavs, and Jews from Southern and Eastern Europe prompted a new nativist upsurge. By the 1920s, the Ku Klux Klan had millions of members. But in the 1930s and 1940s, this wave of nativism largely subsided. What happened? “I argue that the nativists won,” Bennett told me. New federal legislation in the early 1920s closed the “golden door” and shut off the spigot of migrants.

Many have argued that fear and nativism in politics are driven by people’s economic insecurity, as struggling members of the majority find themselves in competition with immigrants for jobs and wages. But Bennett does not believe that to be the case. Nativism, he notes, was relatively low during the Great Depression, and rises in nativist sentiment haven’t generally correlated with periods of economic strain. Rather, they have correlated with large-scale increases in foreign immigration, which natives tend to view as a threat to the nation’s safety and culture. (Recent studies have also found a strong correlation between increases in anti-immigrant sentiment and increases in immigration.) It’s not desperation that makes people turn on the other—it’s diversity.

Right now, America’s foreign-born population is at a historically high level due largely to the surge in Latin American immigration of the last couple of decades. But as some conservative writers have noted, with both the Republican and Democratic establishments officially pro-immigration and pro-diversity, people’s anxieties about this fact had little expression in mainstream political discourse—until Trump came along.

Another form of fear also runs through American politics in the 20th century: the fear of foreign ideology, from anarchism to fascism to Marxism, that solidified into the Cold War fear of communism. Bennett believes that Trump has combined the fear of foreign ideology with fear of foreign immigration in a novel way, with his twin emphases on Islamist terror and Mexican migrants. This, he says, may be why Trump has done better than many fear-fueled politicians.

I asked Bennett if he believed appeals to fear had the power to realign American politics. “These fear-based movements have tended to be confined to the fringe, not take over major political parties,” he said. But the fact that Trump is the Republican nominee makes him wonder whether that historical pattern still holds. “This is what’s making me so nervous,” he said. “I don’t think we know.”

There is a final punch line to the analysis of Trump as the candidate of fear. His opponent, Hillary Clinton, is now campaigning on a fear-based appeal of her own—the fear of Trump.

Clinton’s speech accepting her party’s nomination presented her as the candidate of hopefulness and pluralism, a contrast to Trump’s gloom and doom. But it also sought to sow alarm about the prospect of a Trump presidency, depicting him as erratic and thin-skinned, apt to start a war on a whim. “A man you can bait with a tweet is not a man we can trust with nuclear weapons,” she said. At every turn, she and her aides have portrayed Trump as a “risky” choice with a “temperament” that could lead to dire consequences. It would not be surprising to see her campaign cut an updated version of the “Daisy” ad.

In research conducted for MoveOn, Shenker, the linguistics consultant, found that the idea of Trump as a threat was the most persuasive case against him among swing voters. “The single most damning case against Trump, across the various measurements and using his own words and actions as evidence, is that as President he would escalate the likelihood of catastrophic violent conflict from without and within, posing a serious threat to the future of the United States,” her team wrote in a memo outlining their findings. This message, they noted, was far more effective than emphasizing Trump’s “misogyny” or depicting his economic record as bad for working people.

But Shenker told me that she worries that the Clinton campaign has not done enough to offer a positive vision as an alternative to Trump’s alarmism. “Every time Clinton says, ‘Trump is dangerous,’ what people are hearing is, ‘The world is dangerous, it’s dangerous, it’s dangerous,’” she told me. “It just plays into the message of chaos.” And the more chaotic the world feels, the more people may look to Trump for comfort.