If GOP is truly nervous about mail ballot fraud, there is an easy solution that works for everyone.
Make certain there is full transparency on administrative decisions to accept or reject mail ballots. Make certain that problems can be detected and addressed immediately. 1/

2/ If the parties, candidates, campaigns have full visibility, they can stop problems and follow up with their voters to clear up errors. Ensure that valid votes are counted, and fraudulent votes are rejected.

GOP should insist on transparency.

3/ It solves the problems GOP claims to fear. But today, much is done with mail ballots behind closed doors.

Dems need to insist as well, and train their observers to detect improper actions by voters and election officials.