Michael Bates runs the numbers on Oklahoma Republicans, and concludes that if their first choice is not John McCain, regardless of their actual preference they should vote for Mike Huckabee:
If you’re an Oklahoma Republican and want Mike Huckabee to have a chance at the nomination, vote for Huckabee. You won’t be accidentally helping McCain.
If you’re an Oklahoma Republican and want Mitt Romney to have a chance at the nomination, vote for Huckabee, even if you don’t particularly like Huckabee. Huckabee has the best shot at denying McCain the delegates and the win here in Oklahoma and thus at slowing McCain’s national momentum, which would give Romney the opportunity to fight on.
If you’re an Oklahoma Republican and you don’t like anyone left in the race this is my category vote for Huckabee. Denying McCain a win here helps to stop his momentum and leaves the door open for a new candidate to be chosen at the convention.
It appears that this might work in other states where Huckabee is running second to McCain, as the polls say he is in Oklahoma. And Bates offers a generalized version:
It comes down to this: If you don’t want McCain to be the nominee, you need to vote for the non-McCain candidate who has the best poll numbers in your state.
Emphasis in the original. I am not a Republican, but should McCain prevail, I’ll have no trouble voting against him in November.