Millions of religious voters who showed up to vote for Bush didn't show up for McCain. Karl Rove claimed that could have made up half the margin of defeat.
The SSM law is purging conservative Republicans from town clerk positions in New York, with the open approval of Gov. Cuomo. If the Republicans won't stick up for conservatives on this, that's one less reason to bother voting for them.
That doesn’t explain the losses in Congress at the same time, though. Even if they had abstained from McCain, they would still have supported their senatorial and House candidates... and they still lost.
Also, Rove is not to be trusted.