Mail Voting

The invaluable National Conference on State Legislatures has information on state vote-by-mail procedures.

Reid J. Epstein and Stephanie Saul at NYT:

As with false claims by Republicans about vote-by-mail fraud, there is no evidence to back up the argument from the right that all-mail elections favor Democrats. But Mr. Trump and some of his allies are warning that vote-by-mail poses an existential threat to their party, in hopes of galvanizing Republican opposition to a voting method that is widely seen as safer than in-person voting in the era of the coronavirus.

Five states — Colorado, Hawaii, Oregon, Utah and Washington — now have all-mail elections, in which ballots are sent to every registered voter without their having to request one. Others, like Arizona and California, allow voters to add themselves to a permanent list of mail voters.

And there are also cases like Nebraska, which allows counties of less than 10,000 people to mail ballots to all voters (many of them Republicans) but forbids it in large urban areas (where many voters are Democrats). Texas allows no-excuse absentee voting for people 65 or older, another group that skews Republican.
...
Amelia Showalter, who was the data analytics director for President Barack Obama’s 2012 campaign, found in deeply reported studies of all-mail elections in Colorado in 2014 and Utah in 2016 that there were very slight partisan advantages in each race. [Pro-R in CO, Pro-D in UT)

For one thing, Republicans as well as Democrats engage  in ballot-harvesting campaigns.