Voter purges, apparently.
According to this Daily Kos diary, the same Republican-friendly firms that purged the Florida voter rolls of reliable Democratic voters (read: blacks) are doing the same thing to reliable Democratic voters in Ohio (read: the poor.)
Supposedly the purge was deliberately set up to catch as many Dems as possible, and the "please confirm" letters were deliberately made to look like simple information, when they had an (obscured) action element that required people to go register or they wouldn't be able to vote on election day.
(Well, wouldn't be able to vote without provisional ballots. Guess which districts aren't going to be given enough of those? If you guessed "Republicans", you'd be wrong!)
That's why Rove is so confident. Sure, the polls aren't helping him, but if this is widespread, what do polls matter? He'll win anyway. Sure, people might complain, but he just needs to trot out a few pissant cases of voter fraud and he'll be able to innoculate himself against the charge of widespread voter suppression.
Yet another reason why I've developed a healthy respect of the Canadians. At least their system works.