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Ahead of the 2020 election, everyone wants to know one thing: What will make the difference between Donald Trump winning or losing a second term? Polling suggests that health care, the economy, and immigration are the issues voters care most about. And for many Americans, the erratic and destructive nature of Trump’s presidency itself is a galvanizing force. But there’s another factor that has barely made headlines—the potentially potent influence of the overseas vote.

In 2016, for instance, there were roughly three million Americans living abroad who were eligible to vote, according to data from the Federal Voting Assistance Program (FVAP). Yet in the presidential election, only seven percent of these voters actually cast a ballot.

It’s not clear what states those citizens are registered in, but while Hillary Clinton won the popular vote by three million votes, she lost by thin margins in swing states like Michigan, Wisconsin, and New Hampshire. In other words, the 2.7 million voters abroad who sat out the last presidential election had the power to make a difference.

This year, those voters may not be as hands-off. Experts cite a number of factors that explain the historically low participation rate of overseas voters—like complicated registration systems and a lack of government outreach—but the 2020 election could be different for the simple fact that the reality of a Donald Trump presidency will inspire more Americans abroad to vote. In the leadup to the election, the Democrats have an organized effort to mobilize Democratic voters living in other countries, whereas the GOP doesn’t have a comparable organization.

Democrats Abroad is the party’s official arm abroad. Every four years, it holds a “global primary” and sends delegates to the Democratic National Convention. Significantly, the group is virtually the only voter turnout drive focused on targeting liberal Americans living outside the United States. Its goal in 2020: to get a million overseas voters to cast a ballot.

That’s not an idea that comes from nowhere. Overseas voters have helped decide an election before. In 2000, both George W. Bush’s and Al Gore’s campaigns hinged their victories on a small number of absentee ballots in Florida that were mailed from outside the U.S.

As the New York Times reported in 2001, when Bush’s unofficial lead was around 300 votes at the start of the 18-day recount, the two camps relied heavily on overseas absentee ballots. Bush’s campaign tried to validate the highest number of ballots possible in counties he had already won while seeking to disqualify overseas ballots in counties Gore was leading in.

It seems to have worked. The Times found that when faced with intense pressure from the GOP, Florida officials “accepted hundreds of overseas absentee ballots that failed to comply with state laws.” The analysis of 2,490 votes found that Florida accepted 680 questionable votes—either from ballots without postmarks or postmarked after Election Day; ballots mailed from U.S. cities and states; or even ballots from voters who voted twice. Bush ultimately won the Florida contest by 537 votes.

Tight races like Florida’s send a message to Democrats Abroad, said Julia Bryan, the group’s global chair. The Sunshine State has a 0.5 percent margin for electoral victory, so races can be decided by an incredibly small number of ballots. In 2020, Bryan said the organization wants to “make sure Florida is blue” and help Democrats win tight-margin races in Arizona, Michigan, Wisconsin, and Ohio. “We are the margin of victory for these tight races in states like Florida,” she said.

But there is one problem—there’s no reliable, authoritative data for overseas voters, making the work of mobilization all the more difficult. The World Bank estimates around two million Americans live abroad; a State Department report from 2015 places that number at nine million. FVAP data falls in between—at about 5.7 million. There are also discrepancies when it comes to how many ballots are cast: FVAP—which bases its data on statistical modeling—estimates 200,000 ballots cast in the 2016 election, but data from the Election Administration and Voting Survey (EAVS) is higher, at 380,000.

“That’s the big challenge: there’s no one national, one central database or collection of overseas Americans that is freely shared,” FVAP Director David Beirne told me. “That’s just the nature of the beast.”

For a long time, many observers assumed overseas voters—especially those deployed in the military—leaned Republican, said Amanda Klekowski von Koppenfels, a reader in migration and politics at the Brussels School of International Studies of the University of Kent. More recent numbers, however, may suggest a more Democratic tilt. In North Carolina, a state with a substantial military population, 44 percent of overseas absentee ballot requests were from registered Democrats in 2016, while only 21 percent came from registered Republicans.

At just seven percent, the overseas voting rate in 2016 was minimal. Even then, it had risen two percent from 2012, when only five percent of eligible voters cast a ballot. FVAP estimates the 2014 voting rate at an even lower number—just four percent. Bryan and Klekowski von Koppenfels suspect that turnout will increase again in 2020, which will likely benefit Democrats. “We’re living in countries where we expect to have health care for all,” Bryan said. “This is just normal to us, it’s everyday life. Even the Republicans I know in Prague would never want their health care taken away.”

All in all, there are millions of Americans abroad who could potentially be a deciding factor in ousting Donald Trump. They may just be the Democratic Party’s secret weapon in 2020.

The issue of the overseas voters first gained traction on Capitol Hill in the 1960s, when a group of American expatriates in London and Paris fought for their re-enfranchisement. Congress ultimately granted overseas citizens the right to vote absentee in 1975, after members of the military stationed outside the U.S. had been granted the same right some years earlier. Then, in 1986, lawmakers unified both sets of laws into the Uniformed And Overseas Citizens Absentee Voting Act.

The legislation was overwhelmingly bipartisan. Michael Hanmer, a professor of government and politics at the University of Maryland, said a reason for this is the legislation’s focus on military voters. “It’s next to impossible to find public officials who are going to do things that might disenfranchise members of the military,” Hanmer said.

Today, overseas registrations and ballot requests are completed yearly through an online form and allow citizens to vote in the state where they last resided, regardless of whether they plan to return. Thirty-eight states also allow for citizens who have never lived in the U.S. to vote based on their parents’ last residence.

Beyond Democrats Abroad, there are non-partisan organizations that are trying to ensure Americans living overseas are exercising their constitutional right to the franchise, such as the U.S. Vote Foundation and American Citizens Abroad.

Still, there is only one group really engaged in the rough-and-tumble of partisan politics overseas. This summer, DA will send 21 delegates to the Democratic National Convention in Milwaukee as members of the 51st state. In 2016, one of its delegates was Larry Sanders, who lives in the U.K. and cast his ballot for his brother, Vermont Sen. Bernie Sanders. The Senator himself has participated in virtual town halls for DA, as have nine of the other candidates. But Sanders appears to be popular abroad. Among the nearly 35,000 overseas Democrats who cast their ballots in the 2016 primary, 69 percent sided with Sanders while 31 percent voted for Hillary Clinton.

Bryan has said one of the best ways to ensure Democrats overseas vote is to first get them to join her organization as members. (The group’s membership rate doubled after the 2016 election, she said.) One of these voters is Rebecca Alvarado. A graduate student at the University of Heidelberg, Alvarado was born in Germany to American parents. She’s eligible to vote in her father’s home state of Colorado, which she’s been doing since 2012.

Alvarado attended her first Democrats Abroad meeting in early February. Attendance, she said, had more than doubled from the previous one, which the organizers chalked up to the Iowa caucus fiasco earlier that week. “I finally felt like I can get involved and do something,” she said, “because up until now it’s just been me and a ballot box, or not even a ballot box, me and the post box.”

Notably, the GOP doesn’t have an official arm dedicated to supporting overseas voters—or increasing voter turnout. Republicans Abroad, an organization similar to DA, ceased operations in 2013, and was replaced by Republicans Overseas, which mainly works to raise awareness of the tax situation Americans find themselves in abroad, said Solomon Yue, the organization’s vice-chair and CEO. Voter outreach for the GOP is handled country-by-country.

The Democrats, however, are taking a different approach. Bryan said her team is working to activate American voters across the world. For Democrats, getting rid of Trump is a priority. It may take a global effort to make it happen.