re: Seems one party roundly supports mail in voting and one party opposes

One party relies on the votes of a massive number of dumbasses, who are too ignorant to begin to understand what's in their own best interests. It is much easier to just collect their paperwork and mail in their votes than it is to wake them up, figure out which precinct they vote in, and haul their useless asses to the polls.
Early voting accomplishes the same objectives for that party, but mail-in voting allows them to frick the system right on up until election day.