The best chance of defeating McCain would be for Romney and the Huck to work together. I know the Huck won’t quit, but if Romney were to quit and endorse the Huck, McCain would be toast.
Actually, he sort of did endorse him: "We need to make sure that we have a strong person who can take the baton from President Bush, and Gov. Huckabee is certainly one of those individuals. He'd make a fine president." - Mitt Romney, 2005