Why is mail-in voting a contentious issue? Do you vote by mail?

Just like seemingly everything else in this country, the issue of mail-in voting is cleanly divided along party lines. Democrats support expanding mail-in voting, Republicans oppose it. What makes it a partisan issue? Does mail-in voting favor Democrats? If so, why?

Do you vote by mail and do you support mail-in-only elections?

I have no opinion on this issue right now. I have voted by mail in the past, but I have no preference for in-person voting or mail-in voting.