The “crisis” of illegal immigration has become an article of faith in American  politics.

On the right, Republicans are jockeying over who will lock down the Southern border. “We have people pouring in. They’re pouring in,” Donald Trump says in his latest ad, conjuring an image of invasion that has his supporters dreaming of the “beautiful” wall he has so often promised to build.

The Obama administration disputes Trump, and yet to the astonishment of its Democratic allies, the Department of Homeland Security has relaunched raids to detain and deport hundreds of Central American families that entered the country in the last two years.

So if the situation is that drastic, why does the population of undocumented immigrants in the U.S. keep falling?

That number dropped to 10.9 million in 2014, according to a study published this week by the demographer Robert Warren of the Center for Migration Studies. The population of undocumented immigrants has now fallen every year since 2008, Warren has found, and 2014 marked the first time in a decade that it dropped below 11 million. When Barack Obama took office, the number of undocumented immigrants stood at about 12 million. (Warren’s analysis of Census Bureau data is similar to findings by the Pew Research Center, although Pew’s data suggests that the decline may have leveled off.)

The new report, immigration analysts say, is just the latest indication that the rhetoric on the campaign trail is not matching reality. “There’s a serious disconnect between people preying on public fears and spreading misinformation and what we actually know to be happening,” said Tom Jawetz, the vice president of immigration policy at the Center for American Progress and a former Democratic policy staffer at the House Judiciary Committee.

The downward trend has even come as welcome news to advocates for tighter immigration laws and enforcement, including those who are among the most strident opponents of granting legal status to those already in the U.S. “It’s good,” said Roy Beck, executive director of Numbers USA. “It does suggest that the situation is not as urgent as it was,” he added, referring to the years between 1990 and 2008, when the population of undocumented immigrants increased significantly.

Figuring out exactly why the numbers have fallen is trickier, partly because tracking undocumented immigrants is difficult to begin with. They are, after all, “living in the shadows.” Have more people left the U.S., died, or gained legal status in recent years? Or is the drop primarily due to fewer people coming in? Theories abound, and there are some factors that generate more agreement among immigration analysts than others. For starters, there was the Great Recession. The collapse of the U.S. economy in 2008-2010 played a big part in reducing the flow of border-crossers, since it dulled the jobs magnet that had attracted immigrants in the first place. As the U.S. economy has strengthened in the last few years, the pace of the decline has slowed, but it has not reversed as some had predicted it would.

The other huge factor is Mexico, where a declining birth rate and an improving economy likely have contributed to a drop in the Mexican-born population of undocumented immigrants living in the U.S. by 600,000 since 2010, according to Warren’s study. “The longterm trends in Mexico are driving a lot of this story,” said Randy Capps, director of research at the Migration Policy Institute. A combination of fewer Mexicans entering the workforce and more jobs available at home limits the incentive to venture north. The government has reported that the number of apprehensions along the border has recently dropped to the lowest level in three or four decades. “The flow across the southern border is far, far smaller than it was in previous years” Jawetz said.

The immigrant population would be even lower if it were not for the surge of migrant workers coming from Central America in recent years. While the number of undocumented immigrants from Mexico decreased by 7 percent between 2010 and 2014, the number from Central America rose by 5 percent, Warren found. And his study does not account for a second wave of Central American migrants reported by the federal government in late 2015, which contributed to the Obama administration’s decision to step up its efforts to capture families who had not been granted asylum. But although the influx of women and child migrants has generated a lot of publicity and unease, the vast majority of them have presented themselves to immigration officials at the border in an effort to gain asylum from the violence plaguing their own countries of Guatemala, Honduras, and El Salvador. Unlike many of the Mexican immigrants who enter illegally, “they’re not slipping across the border between ports of entry,” acknowledged Steven Camarota, research director at the Center for Immigration Studies, which advocates for lower overall immigration.

A bigger source of debate over why illegal immigration has fallen is whether the Obama administration deserves any credit for a drop that has happened on its watch. Critics in the GOP have assailed the president for refusing to enforce immigration laws, a policy approach that they say culminated in his 2014 decision—thus far blocked by the courts—to shield millions of immigrants from deportation. “I don’t think [Obama] has done anything to  cause this to happen,” Beck said. He and Camarota noted that while the official unemployment rate in the U.S. has fallen steeply, the labor-force participation rate remains near record lows, and sectors like construction that hire a lot of undocumented immigrants have not fully recovered to their pre-recession levels.

Yet Jawetz and other analysts pointed out that additional money and new technologies deployed at the border in the last several years probably have had an effect. And the Obama administration’s more aggressive enforcement during the president’s first term led the National Council of La Raza to denounce him as the “deporter-in-chief.” Tighter border security could make it more expensive for migrants seeking help from smugglers to get into the United States. And analysts on both sides of the debate said that the harsh anti-immigrant rhetoric emanating from some conservatives could have a small deterrent effect as well.

Under normal circumstances, news that the number of undocumented immigrants was dropping might create the political space for a long-awaited overhaul of immigration laws. The overheated presidential campaign suggests that won’t be happening anytime soon. Even among advocates who acknowledge the downward trend, there isn’t much movement. Beck argues that just because the total number of undocumented immigrants has dropped doesn’t mean that tens of thousands aren’t still coming in every year, presenting both economic and security concerns. “Any amount of illegal immigration,” he maintains, “is harmful to the workers of this country.”