National Journal’s Alex Roarty looks at national polling and polling from South Carolina and Florida and writes of Bachmann, Santorum, and Perry:

But most importantly, their struggles are crippling Romney’s chances. After Herman Cain’s collapse, Bachmann, Santorum and Perry were the three Republicans left in the race capable of attracting the party’s hardline voters. Thus far, they’ve only continued to cede support to Gingrich…Romney needs one of the three to cut into that voting bloc.

In Iowa? Could happen. After that? If Bachmann, Santorum, and Perry don’t finish top three in Iowa, they’re not going to be around for South Carolina (okay, I suppose it’s possible one of them could rally into a strong fourth and win the spin game and therefore “win” Iowa, but basically they need to be top three).

Quick review: look at the percentage of the overall vote taken by the top two candidates, Iowa vs. South Carolina (all numbers from wikipedia):

             Iowa    SC
1980      62%    85%

1988      62       70
1996      49       74

2000      72       95
2008      59       63

So in each case the percentage of votes for other candidates went down, sometimes just a bit but in some cases dramatically. And, of course, it continued to decrease after that. Most losing candidates don’t just go from 10% to 10% to 10% as the campaign goes on; if they have no shot at the nomination, they drop out. Indeed, of the current contenders, only Ron Paul seems likely to stay in after proving he doesn’t have the electoral support to have a realistic shot at the nomination.

It’s just not realistic to expect the also-rans to soak up a lot of votes beyond the first handful of events.

And of course the flip side of this, as I’ve started to point out regularly, is that there’s no guarantee that Romney will wind up surviving the winnowing process. If he finishes 4th (or worse) in Iowa, he may well fall to second or third in New Hampshire, and be gone before Supertuesday. So if I were running Mitt Romney’s campaign, I wouldn’t exactly be rooting for a Perry or even a Bachmann surge right now.

At any rate, given this cycle’s cast of characters and schedule, I find it hard to believe that we’ll have more than two candidates plus Ron Paul after Florida, so most of the delegates will be chosen in states in which well over 40% will be needed to win.

The bottom line is that if Romney really has a hard cap below 50% he’s almost certainly not going to win the nomination.

[Cross-posted at A plain blog about politics]

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Jonathan Bernstein is a political scientist who writes about American politics, especially the presidency, Congress, parties, and elections.