Mike Huckabee is running for vice-president. His argument is good: he draws voters who McCain cannot reach, he can run for the right while McCain campaigns for the center, he has a different kind of charisma than McCain and campaigns well. So he is in a position to offer himself for the post, something neither Romney nor Giuliani nor Thompson can credibly do. But he's not yet at the point where he can shoot his way on to the ticket, the way George H. W. Bush and James Baker did in 1980, by winning enough delegates to go to the convention with an offer too good to refuse. To do that, Huckabee would have to go in to the convention with at least 25-30 percent of the delegates. I don't know if he can do that, but I think he's very well-positioned to be the veep nominee. McCain could of course choose someone else and survive, but that would be a costly gesture all other things being equal.
Meanwhile it looks like this same basic argument from general election expediancy pushes strongly for a woman as veep on the Democrats' side. I would have said some months ago that either Hillary or Barack would have to go with some senior white male Democrat to assuage voters wary of a black/woman, but it doesn't turn out that way: for practical political purposes, if he wins the nomination Obama will have to have a woman for veep (even though it won't be Clinton).