I dont know who benefits from mail in voting more. Arguably, trump benefits as mail in voting would help more older voters vote, which was until recently skewed in trumps favour. So why are democrats pushing for it so forcefully? It's asking for trouble on many fronts. All i can think is they think they can cheat.
Actually reading some more articles about it, I might be wrong here.
https://www.nytimes.com/2020/05/25/us/vote-by-mail-coronavirus.html
https://fivethirtyeight.com/feature...-voting-by-mail-gives-one-party-an-advantage/
Conventional wisdom says increased turnout = more votes for dems. There's just naturally more democrats in the country. Even some solid red states like Kentucky are majority dem by voter registration. So compulsory voting would likely ensure a big dem win. But mail-ins are a bit more complicated than that and doesn't necessarily increase voter participation, and there are so many factors going into this here, especially with the unprecedented nature of our current situation.
Maybe the party sees specific reasons to believe it will give them a boost. I just don't think cheating would be likely or even necessary, considering they have a pretty solid lead right now in the polling. One issue may be that Trump naturally has a more enthusiastic base, and may also have more people willing to take risks during covid. So pushing for a more in-person election and making mail-ins more difficult could leave a lot of dems staying home while republicans don't care and go to the polls.