There are two, primary things that have fueled Mitt Romney's campaign -- his image as an economic guru and the idea that he's the most electable of the Republican nominees.
But there are joint arguments that call the latter into question.
First, there's the theoretical argument -- made today by Red State's Erick Erickson in a blistering, bruising post -- which claims that Romney is a certain loser against Barack Obama.
Mitt Romney, on the other hand, is a man devoid of any principles other than getting himself elected. As much as the American public does not like Barack Obama, they loath a man so fueled with ambition that he will say or do anything to get himself elected. Mitt Romney is that man.
.... To beat Barack Obama, a candidate must paint a bold contrast with the Democrats on their policies. When Mitt Romney tries, Barack Obama will be able to show that just the other day Mitt Romney held exactly the opposite position as the one he holds today.
Voters may not like Barack Obama, but by the time Obama is done with Romney they will not trust Mitt Romney. And voters would rather the guy they don’t like than they guy they don’t trust.
There's some evidence that Erickson is right.
Karl Rove, whom most peg to be quietly backing Romney, has made the same point -- except in the context of John Kerry; not Romney.
In the 2004 election, most Americans stood on Kerry's side of the issues, but Rove claims they ultimately voted for Bush because they didn't really believe Kerry believed anything. Voters supposedly like strong leaders they disagree with better than weak leaders who might agree with them on Monday but wake up on Tuesday, wearing a different face.
That's exactly the argument Erickson is making, and it's precisely the one that could hurt Romney badly.
Now here's the empirical argument.
A new WSJ/NBC poll (pdf) shows that a 3rd party bid by a strong libertarian/tea party challenger like Ron Paul would be disastrous for the Republican cause next November.
Barack Obama decisively beats Romney in a three-way matchup with Paul, 44%-32%. That's because Paul wins 18% of the vote.
It's safe to say that, besides Jon Huntsman, a Romney nomination would have the greatest chance of inspiring such a third party bid. thereby, dooming the Republican candidate and reelecting Obama.
Now, the counter to that (which I buy) is that Paul actually wouldn't pull in 18% of the vote -- that when push came to shove, lots of folks who hate Obama and hate Romney only slightly less would grudgingly vote for Mitt.
But it does put Romney's electability argument in an entirely new light.