Nothing illustrates the mixed-up politics of vote-by-mail better than the world’s most famous absentee voter declaring the practice corrupt. “Mail ballots are a very dangerous thing for this country, because they’re cheaters,” Donald Trump told reporters in early April, a few weeks after casting an absentee ballot in Florida’s primary. “They’re fraudulent in many cases.”

With the timeline of the coronavirus pandemic still uncertain, it’s clear that the US needs a plan to allow people to vote in November without putting their lives at risk. The most obvious option is to let everyone cast their ballot by mail. But, as with so many things coronavirus-related, that idea has become polarized along partisan lines. A poll released this week by the Associated Press-NORC Center for Public Affairs Research showed 73 percent of Democrats in favor of allowing everyone to vote by mail, compared to just 46 percent of Republicans (and 59 percent of independents). In Washington, while congressional Democrats push to include funding to expand vote-by-mail in the federal coronavirus relief bills, some Republican officials, most notably Trump, decry the idea as a partisan power grab. “They had things, levels of voting that if you’d ever agreed to it, you’d never have a Republican elected in this country again,” Trump told the hosts of Fox & Friends in March, referring to the Democrats’ proposals.

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Trump was roundly ridiculed for suggesting that expanding vote-by-mail would hurt Republicans in November. The New York Times called it a “false claim,” declaring that “there is no evidence to back up the argument from the right that all-mail elections favor Democrats.” But the truth is a little more complicated.

Universal Vote-by-Mail Raises Turnout—but for Whom?

There are basically three categories of vote-by-mail in the US. The most restrictive level, found in seven states, is traditional absentee balloting, where voters have to give a reason why they can’t vote in person. Next is no-excuse absentee, where anyone can vote by mail, but must request a ballot. About half of states have a version of that. Then there’s universal vote-by-mail, or “vote at home,” a system now used in five states—Colorado, Hawaii, Oregon, Utah, and Washington—plus many counties in California. The government automatically mails a ballot to every registered voter, and voters have about two weeks to either mail the ballot back or drop it off in person by election day.

Most of the research focuses on that last category. Vote at home made its general-election debut in Oregon in 2000 and five years later spread to neighboring Washington State, where all but one county quickly opted in. (The last holdout joined in 2012.) Those two states now consistently have some of the nation’s highest turnout rates. So does Colorado, which adopted the system in 2014. Studies have long found a link between vote-by-mail and higher turnout. It’s easy to see why: when voting becomes more convenient, more people tend to do it.

Bringing in more voters is not an unalloyed good in the eyes of all politicians, however, so support for vote-by-mail has always had partisan twists—but which party is in support has varied over time and across states. In 1995, Oregon’s Republican-controlled legislature passed a bill enacting vote at home, only to have it vetoed by governor John Kitzhaber, a Democrat. (Three years later, voters overwhelmingly approved the change through a ballot initiative.) Many Democrats worried it would help the other side. Absentee voting has long been a favorite of older, whiter, Republican-leaning voters. Phil Keisling administered the transition to vote at home as Oregon’s secretary of state in the 1990s, and now chairs the National Vote at Home Institute, a nonpartisan organization that advocates expanding the reform. He recalled conversations with Democratic politicians who worried, “It’ll help rural white voters, and it won’t help our base, and it’s a bad idea.” Skittish Democrats saw confirmation of their fears in 2014, when, in Colorado’s first all-mail election, Republicans wiped Democrats out in statewide races.

Recent research has pushed the pendulum in the other direction. A 2017 study commissioned by the Washington Monthly, where I was an editor, and conducted by Amelia Showalter of Pantheon Analytics found that Colorado’s new system appeared to account for a 3.3 percent overall turnout increase in 2014, compared to a turnout model that was highly accurate but didn’t factor in new voting systems. That bump was overwhelmingly concentrated among younger and lower-propensity voters: the 18-24 year-old bracket beat the model by 12.1 percent. In another study, Showalter looked at Utah, which had been expanding vote at home county by county since 2012. This created a natural experiment. Showalter isolated voters clustered on either side of county lines and compared turnout changes between the ones who had vote at home and the ones who didn’t. The result: traditional counties slightly underperformed the turnout model, while all-mail counties outperformed it by wide margins. The boosts were again more pronounced among young voters.