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.