Look at 1992 or 1996 for the answer to that.
Some will vote for a third party for President, or not vote for that office.
Others will stay home.
A few will vote for the RAT candidate, figuring they'd rather have a real democrat than a pretend one.
It's a political calculation, and that's what Rove is really good at.
It's pretty gutless for Bush, but there's an election to win, I guess.