As a non US citizen, based on how aggressively the the republican party seems to play with voter suppression it wouldn't seem unlikely that there won't be vote by mail in the next election.
They are, but what can a state government do if the administration directs USPS to refuse ballots? Even if they file a lawsuit and win an injunction, should voters or state election officials trust the administration to fully comply?
At lot of things which are well-established norms and procedures are being thrown out at a record pace with this administration, why would this be any different?
Party? Probably not. However, it would favor the working class as not everyone can take a day to go to the polls. It also helps the elderly, medically fragile, and others who otherwise have trouble getting to the polls. Nevermind the voters who happen to be traveling abroad and want to cast their votes.