You’ve probably noticed them—those celebrities on stage at campaign events, creating hype for presidential aspirants who may or may not be more charismatic than they are. Or those House and Senate members taking to cable news to assert their chosen contenders’ bona fides. Candidates depend on these individuals, called surrogates, to lend enthusiasm and credibility to a campaign. But depending on their level of expertise—or personal behavior—their usefulness can vary.

This cycle, surrogates fanned out across early primary states to help their favorite politicians and haven’t stopped since. They don’t just give their thumbs-up to a candidate—surrogates serve practical functions, too, filling in when a candidate cannot attend a particular speech or fundraiser, or appearing alongside the candidate in a show of support. Campaigns make innumerable calculations about how to invest candidates’ hours, and surrogates take away “some of the burden,” said Eric Kasper, a political science professor from the University of Wisconsin. There hasn’t been much research to quantify surrogates’ influence, but they certainly don’t “win or lose elections,” as Dan Schnur, a former Republican campaign-communications official, put it last cycle. They are more likely to help in the margins, he said. Still, in a competitive campaign, those marginal victories can add up, and if surrogates can help candidates even a little, campaigns are going to keep using them.

All surrogates are not created equal. Sometimes, surrogates can bring both political savvy and celebrity to the table, and candidates have different needs at different times. When Clinton wanted to bring gun control to the fore of national debate, she brought mothers whose children were killed by gun violence on the trail. Ahead of the Iowa caucuses, the Sanders campaign dispatched big names like Killer Mike and Cornel West in lieu of politicians. Kasper said Trump could find both his celebrity and political surrogates helpful going into November: the celebrities, because they might appeal to disaffected voters, and the politicians, because they might help him court the traditional Republican voters he needs to win.

Potential first ladies are often deployed to help Americans see their mate through their presumably affectionate eyes. Ditto candidates’ children. Other individuals can be used to target a particular community of voters or speak to a specific policy area, said Lara Brown, the program director for George Washington University’s graduate-level political-management program. If House Minority Leader Nancy Pelosi, for example, were to endorse Hillary Clinton, she could “attest” to Clinton’s credibility with women and girls, Brown said, because she is ostensibly knowledgeable about those communities’ issues.

The potency of surrogates from the political world can depend on “how close that surrogate is to either the candidate or how high up they are within the party,” Brown said. When President Obama stumps on the trail for his party’s nominee, it will matter “because he’s not only the sitting president, but he’s essentially the titular head of the party,” she said. Celebrity surrogates have different advantages: They can help campaigns attract eyeballs and event crowds, Kasper suggested, particularly among younger or disengaged voters. They are typically more effective at raising money than lending issue-specific gravitas, Brown suggested; they are not as helpful as, say, an elected official in the policy arena. A few have it all: Former President Bill Clinton hits three surrogate categories, as a political spouse, fluent policy wonk, and celebrity in his own right.

Even when surrogates try to stay on-message, though, they can make unintended mistakes. In recent weeks, Donald Trump’s congressional surrogates have had difficulty defending or explaining his ideas in interviews, MSNBC reported, because his “day-to-day pronouncements are unpredictable and frequently controversial.” Some voters bristled at Clinton surrogate Madeleine Albright’s suggestion earlier this year that “there’s a special place in hell for women who don’t help each other.” And Susan Sarandon, a Bernie Sanders surrogate, took criticism for her Bernie-or-bust stance.

Surrogates’ ill-advised utterings have long distracted campaigns. “When a surrogate errs, the mistake tends to reflect on the competency and values of the campaign and, by implication, the ability of the candidate to govern,” The New York Times reported in 1989, referring to a dust-up in Rudy Giuliani’s mayoral campaign. The comedian Jackie Mason had said something offensive about Jewish voters, and Giuliani himself had to answer for it. “It's a low-budget way of getting attention, but it’s a high-risk strategy because you're held responsible for what your surrogates say and do,” the Democratic political adviser Richard Moe said at the time.

None of the presidential campaigns responded to questions about their surrogate operations, but a January CNN report offers a peek into Clinton’s then-five-member surrogate team and Sanders’ smaller effort. Surrogates may seem particularly useful in this social-media- and cable-news-driven age, when candidates’ time is stretched increasingly thin. But these efforts have been around for decades. Brown highlighted the use of surrogates during the Gilded Age, when candidates couldn’t travel as easily as they do now. Party networks were stronger way back when, and political machines had surrogates “everywhere”—indeed, the machines were “essentially” the surrogates. This style of campaigning has had modern influence: In 1996, the Bob Dole campaign modeled its surrogate deployment on that of President William McKinley a century earlier. It was Newt Gingrich’s idea, the Los Angeles Times reported at the time:

Dole, the party’s presumptive presidential nominee, is an admittedly lackluster campaigner compared to [incumbent President Bill] Clinton, who is a polished orator. So the GOP troika and its top advisors are scripting a coordinated effort in which there is one central message but many voices—surrogates speaking in Dole's behalf. … Like this year, the election of 100 years ago pitted candidates who were a study in contrasts.

William McKinley was an older GOP stalwart. In part because he was such an uninspiring speaker, some 1,400 surrogates stumped on his behalf while he remained at home in Ohio, greeting well-wishers from his front porch.

His Democratic opponent was William Jennings Bryan, a young and fiery prairie populist who crisscrossed the nation, often delivering five or six speeches a day and waging the most energetic presidential campaign in U.S. history up to then. But with the help of a united party, McKinley won.

Dole did not end up winning that election, of course. He lost to Clinton by nine points. But in the 20 years since—and 100 years since McKinley—campaigns have continued to use surrogates. They know surrogates won’t win elections, but there’s always the potential surrogates that can offer an edge over a campaign’s presidential competition.