Obviously neither is acceptable.

When McCain talks about "voter fraud" at rallies, what he is saying is that the outcome of the election will be affected. Voter fraud means that votes are cast fraudulently. This is why Republicans are so worked up about this issue right now.

"Voter registration fraud" means that some registration volunteers got paid money they shouldn't have been paid. It has no effect on the outcome of the election, except perhaps indirectly, and that would be to the detriment of the Democrats since I assume ACORN was putting more effort into registering Democrat demographics.

You think the Republicans are so incensed about this scandal right now because they are worried volunteers got overpaid and some Democrat registrations might get lost?

No, they are angry because McCain is spinning this so it appears the Democrats are trying to cheat in the election.