The most basic reason that vote-by-mail as a system favors GOP is that a majority of ppl who don't have a mailing address (and hence can't receive a ballot in the mail) would vote against the GOP if they voted at all. There are a dozens of such ppl I pass on the downtown Seattle streets each day, or did when we used to go to the office to work, and that's true all over the US. To take an extreme example, there are 60,000 homeless ppl in LA County and proportionately similar in all big US cities. The Republican Secretary of State in Washington (only statewide GOP officeholder surviving on the West Coast), Kim Wyman, has tried to explain this to her GOP colleagues for years and years.
The most basic reason that vote-by-mail as a system favors GOP is that a majority of ppl who don't have a mailing address (and hence can't receive a ballot in the mail) would vote against the GOP if they voted at all. There are a dozens of such ppl I pass on the downtown Seattle streets each day, or did when we used to go to the office to work, and that's true all over the US. To take an extreme example, there are 60,000 homeless ppl in LA County and proportionately similar in all big US cities. The Republican Secretary of State in Washington (only statewide GOP officeholder surviving on the West Coast), Kim Wyman, has tried to explain this to her GOP colleagues for years and years.