Numerous studies have arrived at the same conclusion: Voting by mail doesn’t provide any clear partisan advantage. In fact, as states have expanded their use of mailed ballots over the last decade — including five states that conduct all-mail elections by default — both parties have enjoyed a small but equal increase in turnout.
In short: voting by mail is more convenient for some voters but more difficult for others, and these conflicting factors appear to cancel each other out, dampening any partisan advantage. Moreover, the vast majority of nonvoters don’t participate not because it’s too inconvenient to vote, but because voting isn’t a habit for them. Maybe they don’t care about politics, maybe they don’t think their vote matters, maybe they don’t like any of the candidates, or maybe it’s some combination of all of the above. But the bottom line is that these voters’ decision to vote depends more on whether somebody around them can motivate them to vote, not whether they are able to vote by mail or in person.