Democrats did use absentee ballots in 2000. They used them in New Jersey and New York and then went to the polls in Dade, Broward and Palm Beach and voted again. That little tidbit of info never reached the media. The numbers were enough to tip the election. They wanted to make sure it counted.
They wanted to make sure it counted. ....and counted....and counted.
Do you suppose some sub-set of that group with a sense of morality (okay, an admitedly small one), recognizing that they'd already voted in the national races, just cast ballots in the local races? That would produce.... gasp!... an undervote.