Mitt Romney's campaign is out with a new memo explaining why he's inevitable and arguing that the campaign is effectively over, but the mere fact that they had to write the memo is prima facie evidence that they are wrong. It might reflect their hopes and desires, but it doesn't reflect reality.
Obviously, Mitt Romney is the frontrunner—he leads the Republican field in both popular votes cast and in delegates won. There's no question about that, nor is there any question about the fact that he is in the best position to win the nomination. But he's got reason to worry, and if you take a look at the numbers I posted earlier on how popular vote is breaking down, you'll see why.
The good news for Mitt Romney in those numbers is that he's still leading, but the bad news is that his support is shrinking. And if you take Virginia out of the picture (remember that neither Rick Santorum nor Newt Gingrich were allowed on the ballot there), you can see Romney's support is shrinking even more:
Romney's shrinking support isn't his only problem, nor is the fact that Rick Santorum shrunk the margin between him and Romney from
1316 points before Super Tuesday to 7 points on Super Tuesday. Another big problem is that
Mitt RomneyRick Santorum and Newt Gingrich combined have more than fifty percent support. As long as both candidates remain in the race, that's a manageable issue for Romney, but if one of them were to drop out, Romney could find himself trailing a surging Santorum or Gingrich. If that were to happen, Romney's grip on the nomination could easily slip away.
Romney might be able to battle back against a single opponent, and with his resources, perhaps he'd be a favorite to do so. But that's not a risk he wants to take, and that's why he wants to shut the primary down. The thing he needs to be careful of is that his motive is fairly transparent, and if Republican voters get the impression that Romney is trying to short-circuit the process, he's going to face even more trouble, whether or not Santorum or Gingrich pull out.