Cato Institute’s David Boaz gives you the answer.
From Politico’s Arena:
Question: Is Mitt Romney unstoppable?
Romney is at 85 percent on Intrade, and he’s a good buy at that price. His success rather mystifies me. He has only four years in public office, and his only accomplishment there – the health care mandate – is highly unpopular with Republicans, for good reason. This really is the best evidence yet for the hypothesis that Republicans nominate the guy who ran second the last time.
And speaking of second, I think the only candidate who’s done well enough to persist after South Carolina is Ron Paul, and he’s also the candidate with no good reason to drop out. So I’d expect a long Romney-Paul debate. And let’s take note of a particular accomplishment by Paul. One rule in presidential politics is that Americans don’t elect House members to the presidency.
Last night Ron Paul did better in the New Hampshire primary than any other House member in history – better than Morris Udall, or Jack Kemp, or indeed former speaker Gingrich. He’s also the oldest candidate who ever drew noticeable votes in New Hampshire. Ron Paul is a 76-year-old House member, and yet his libertarian message has given him a strong performance in both Iowa and New Hampshire. It gives me a lot of hope that 23 percent is not an upper limit for a libertarian candidate, but rather that a libertarian-leaning governor or senator, in his fifties, without the baggage that burdens Paul, could do far better.