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Voting by mail hasn’t given a big advantage to one political party, but Republican rhetoric could change the dynamic for November’s election.

Alyssa Pointer/Atlanta Journal-Constitution via AP

Do Republicans or Democrats benefit from mail-in voting? It turns out, neither

In the United States, the coronavirus crisis has thrust a typically wonky debate—the effectiveness of mail-in voting—into the political spotlight. Republicans, led by President Donald Trump, this week again warned that expanding the use of mail-in ballots could give Democrats an edge in the November elections. Now, two independent studies suggest there’s little historical evidence to support that fear. But scientists warn that by making vote by mail a partisan issue, Republicans could lose mail-in votes and benefit Democrats in the midst of a caustic and pandemic-marred election season.

Since 2000, a handful of states have switched almost exclusively to voting by mail, including Oregon, Washington, Utah, Colorado, and Hawaii. Most researchers studied the moves to see whether they led to an increase in voter participation; largely unexplored, says Brigham Young University political scientist Michael Barber, was whether universal mail-in voting benefited one political party over another.

That all changed when Republicans warned earlier this year that efforts to expand vote by mail could benefit Democrats. Barber and political scientist John Holbein at the University of Virginia decided to test those claims by comparing voting behavior in counties that switched to universal mail-in voting (175 counties in 2018) with the nearly 3000 counties that didn’t. They also looked at voter behavior before and after each changeover.

They found that in presidential and midterm general elections between 1996 and 2018, switching to all-mail voting increased the percentage of residents who voted by 1.8% to 2.9%, they report today in Science Advances. When it came to the Democratic share of the vote, they found a tiny uptick in the share of votes that went to Democratic candidates for Congress, governor, and president—approximately 0.7%. But the difference was so small that the margin of statistical error means it’s possible there was no effect at all, Holbein says. “There might be a teensy, tiny effect on Democratic turnout.”

To double check their results, the two analyzed 40 million voting records in Washington and Utah—two states that gradually changed to all-mail voting, with some counties switching earlier than others. Although those records don’t show how someone voted, they do show whether they registered as a Democrat, Republican, or independent, and whether they took part in an election.

That let the scientists see whether voters in counties that went to all-mail ballots changed their voting activity compared with counties where people could still go to polling places. And they could see whether people who registered as Republicans acted differently from those registered as Democrats. It also gave them a chance to compare voter behavior in a solidly conservative state, Utah, with Democratic-leaning Washington. Again, they found the switch to mail-in ballots made no statistically significant difference in voting levels by Democrats versus Republicans.

The apparent lack of a partisan effect could be because the increase in turnout triggered by mail balloting is relatively small, Holbein says. For such a small number of new voters to have a lopsided impact, they would need to be overwhelmingly affiliated with one party or another. Holbein suspects the new voters are little different from other voters—but that they simply needed the “nudge” of mail-in ballots.

The new results mirror work published in June by researchers from Stanford University. In that study, scientists looked for changes in voting as counties in Washington, Utah, and California gradually switched to all-mail ballots in 2006, 2012, and 2018. They found a negligible partisan impact, with turnout by registered Democrats increasing by just 0.1% relative to Republicans, they reported in the Proceedings of the National Academy of Sciences.

“The takeaway is really good news: Two independent groups using similar data arrived at the same conclusions,” says Andrew Hall, a Stanford political scientist and co-author of the paper.

Although the research shows that any partisan impact is small, the potential gains by Democrats could prove real on closer scrutiny, says Charles Stewart III, a political scientist at the Massachusetts Institute of Technology who studies voting technology and administration. “I could imagine a future study that had more observations and more statistical power finding that some of these effects were statistically significant,” he says.

Then there’s the question of whether the past has any predictive power for November’s election, which will be held amid a pandemic and with one party criticizing voting by mail. It’s possible, Barber says, that Republican rhetoric could depress vote by mail among their members and cause a partisan split. “The problem is that Donald Trump has created a sort of self-fulfilling prophecy.”

There are already signs that this is happening. In Florida, requests for mail-in ballots are usually evenly split between Republicans and Democrats, says Michael McDonald, a political scientist at the University of Florida. This year, 2 million Florida Democrats had requested mail ballots as of 25 August, compared with 1.375 million Republicans, according to a website McDonald runs that tracks mail ballot trends. Add to that the uneven ways in which mail-in voting is being rolled out in different states, and McDonald is at a loss to predict how it will influence the election outcome. “It’s completely uncharted territory,” he says.

There’s also the possibility that Republican reluctance to use mail-in ballots will be offset by Democrats’ avoidance of polling places. In a Texas primary in June, Democratic use of mail-in ballots skyrocketed. But the increase was canceled out by more Republicans voting in person, according to a preprint study by Hall and other Stanford researchers not yet published in a journal. “They’re voting by different modes,” he says. “But they’re not voting more or less.”

This year’s extraordinary circumstances mean the November election could defy history. “The world has changed so much even since this paper was written,” Barber says. “It’s a very different landscape now than even 6 months ago.”