They'll have to speak for themselves, but I wouldn't be surprised if they stayed at home and sat on their hands. I've only seen one McClintock supporter over the past three weeks of posts here on FR say that if Arnold is ahead by election day, he'll vote for Arnold.
One of the problems is that inspite of arguing from "logic," too many McClintock supporters have invested too much emotion in his candidacy, and that emotion has turned into irrational hatred for Arnold. They have gone too far out on a limb to admit, in the end, that voting for Arnold is the only real option. They've excoriated Arnold too much on these threads. They couldn't vote for him; they have their pride to think about. Now, had they argued from principle, but said from the very start that while they support McClintock, they will vote for whichever Republican is ahead by election day, they'd have room to maneuver. But now they've boxed themselves in.