Harry Enten offers some pieces of evidence suggesting (although not proving) that expanding mail-in voting hasn’t had a strong pro-Democratic effect on election outcomes, and that restricting it wouldn’t have a strong pro-Republican effect. The lesson he draws is that “it’s not at all clear Republicans will succeed in helping their electoral prospects” by enacting new limits on mail-in voting.
If he’s right, though, it also means that the progressive fear that Republicans will entrench their power by limiting mail-in voting is off-base. I recently wrote about this fear, and some other reasons for thinking it’s overwrought.