Knocking off Mitt Romney will be tough, no doubt about it. We've seen Newt Gingrich and Rick Perry take big shots at his Bain record in the last week or so, and the response in defense of Romney, and against Gingrich and Perry, has been fairly wide-spread. There are many forces in the Republican party beside the activist base, and they have a lot of resources at their disposal - a potential rival to Mitt Romney that upsets too many parts of the GOP will see major headwinds in his effort to derail a Romney nomination. Going against Romney's war chest AND drawing the fire of people who aren't necessarily Romney fans but are certainly party stalwarts will be incredibly difficult; a candidate who is deemed acceptable by the most factions of the party would stand the best chance of surviving the showdown with Romney.
In this diary, I'll try to assess each individual candidate's chances of prevailing if the nomination contest comes down to a three-way battle of Romney vs. Paul vs. Candidate X.
Newt Gingrich would probably attract the most blowback were he to be the final non-Romney standing. We know that Congressional Republicans who remember his leadership in the 90's are almost universally against him. Business interests resent him for his Bain rhetoric. Finally, the part of the GOP apparatus that cares most about electability could never bring themselves to believe that Gingrich, with his past history of scandal and partisan image, is more electable than Romney.
Rick Perry doesn't have a past history of annoying fellow Republicans like Gingrich does. However, with his 'vulture capitalist' attacks on Romney's Bain record, he did join Gingrich on the enemies list of a lot of business interests. He also is unlikely to be acceptable to the 'electability caucus' within the GOP, which has already rendered its verdict on Perry, evidenced by his slide in the polls.
Rick Santorum hasn't attacked on Bain; however, he is still somewhat suspect on fiscal issues, with a past history of earmarks, among other things. The Wall Street Journal's Kimberly Strassel and AEI's Kevin Hassett have already editorialized against Santorum's tax proposals; it seems that if it came down to Romney vs. Santorum, Republican opinion-makers who care most about fiscal policy will view him like they did Mike Huckabee, and line up with Romney - and money and super-PAC ads will follow. (Note: I do not mean to debate the actual merits of Santorum's tax policy here.) In addition, with Santorum's social conservatism being so prominent, many in the electability crowd might balk and pull out the stops for Romney. With a small war chest, Santorum would find this a very steep hill to climb indeed.
Jon Huntsman's policies are the favorite of fiscal conservatives, with the Wall Street Journal giving a big thumbs up, compared with their verdict of timid for Mitt Romney. He's not a dyed-in-the-wool social conservative, but being solidly pro-life and anti-gay marriage makes him acceptable to social conservatives, perhaps even more so than Romney, since Huntsman has been consistent in his views. He has proven his appeal to independents and swing voters, so the electability caucus has no beef with him. His biggest problem is that a lot of conservative voters see him as somehow "more moderate" than Romney, in large part because of the ambassadorship. If, however, the primaries end up at Romney vs. Paul vs. Huntsman, there will be a lot of articles explaining that, actually, Huntsman's a pretty conservative guy, which many primary voters haven't yet come to realize. In pretty much every comparison one can make, no matter one's biases, he comes out ahead of Romney, so all of the factions in the GOP will hold their fire, or even support Huntsman (e.g., the Wall Street Journal). Romney's campaign operatives will be the only ones driving an anti-Huntsman narrative, and they don't really have much - Romney has already tried dissing Huntsman "working for Obama" in the New Hampshire debate and gotten smacked back, so what else is there?
One additional advantage - admittedly, a small one - Huntsman would have is that he is probably the candidate best equipped to pick up the handful of Paul voters that might decide to vote tactically in a Romney 40%, Candidate X 30%, Paul 20% race, given his views on winding down the war in Afghanistan and his proposal to reform Too-Big-To-Fail.
In short, I think that Huntsman stands the best chance to take out Romney, if he were to become Romney's main rival. Now, about getting from here to there...