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.”