The conventional wisdom on the Republican nomination race has once again shifted. In the span of just two weeks, Mitt Romney has gone from seeming quite vulnerable to the near-inevitable Republican nominee. The odds attributed to Mr. Romney winning the nomination at the betting market Intrade, which closed at a low of 42 percent on Dec. 13, had shot up to 72 percent as of Monday night.

I don’t know that Mr. Romney’s stock is mispriced — if anything, it might be a little cheap. It’s not that Mr. Romney is all that strong a candidate. But for him to fail to win the nomination, someone else has to, and it’s hard to see who that is.

Newt Gingrich has been moving backward in the polls and would have a lot of hurdles to overcome even if he rebounded. Ron Paul’s numbers have been moving upward, but he’s arguably more helpful than harmful to Mr. Romney and has little chance to win the nomination once the field is winnowed down.

Jon M. Huntsman Jr. has more upside than some of the other Republican candidates, and his polling has improved in New Hampshire. But he’d look a lot more dangerous if he were polling at 21 percent there rather than 12 percent.

Rick Perry is in something of a parallel position in Iowa. He’s someone who could look wholly different to voters with a strong finish there. But for now he’s stuck splitting the evangelical vote with Michele Bachmann and Rick Santorum and hasn’t been able to build up much momentum.

Meanwhile, little things are helping Mr. Romney around the margin. Few of the other campaigns have launched a prolonged attack against him (although it appears that Mr. Gingrich, reneging on a pledge to run a positive campaign, may give it a late effort). All of Mr. Romney’s competitors except Mr. Paul failed to qualify for the ballot in Virginia. Mr. Romney continues to add to his pile of endorsements. Even something like President Obama’s modestly improved approval ratings could play favorably for Mr. Romney if they remind Republican voters that an incumbent president is never easy to defeat and that there is a premium on nominating an electable, mainstream candidate like Mr. Romney.

Still, Mr. Romney’s numbers are not those of a traditional frontrunner. He’s at only about 25 percent in national polls, which is improved from two weeks ago but only barely. He’s still an underdog to win Iowa. His favorability ratings with Republican voters are adequate but not more than that. A lot of Republican voters remain dissatisfied with their choices. Usually, those numbers improve as the actual voting draws nearer, but if anything they’ve been getting worse lately.

This disconnect between expectations and performance is potentially quite dangerous to Mr. Romney. Consider Iowa, for instance. Mr. Romney currently projects to about 22 percent of the vote there, but the history of Iowa is one of volatile polling right up to the last minute — and sometimes huge surprises on election night. Our state-by-state forecasts, which account for this uncertainty, say that Mr. Romney could finish with as much as 36 percent of the vote in Iowa, but also as little as 8 percent, which could drop him all the way down to fifth or sixth place.

That such scenarios are plausible does not mean they are likely. But there is not all that much margin separating the candidates, and Republicans with Mr. Romney’s profile have historically underachieved their polls on caucus night.

Meanwhile, expectations seem to have gotten a little ahead of themselves. “I don’t see any scenario where we’re not the nominee,” one of Mr. Romney’s strategists told New York magazine’s John Heilemann.

Actually, the scenario is pretty easy to articulate. As I frequently remind our readers, the momentum that candidates get out of the early states has historically had as much to do with expectations as the actual results. Even a third-place finish in Iowa, much less something worse, might now be viewed as disappointing for Mr. Romney, increasing the risk of either a loss in New Hampshire or a close call that made Mr. Romney vulnerable heading into South Carolina and Florida.

My view is that the probability of these scenarios is higher than is generally acknowledged. Both the news media and the campaigns are always surprised when things don’t go according to the polls in the early voting states, even though a quick glance at the historical ledger would remind them that this sort of thing happens all the time.

So why am I nevertheless fairly bullish on Mr. Romney’s campaign? Well, there’s still that issue of one of the other candidates actually having to defeat him. One of the more likely scenarios is that Mr. Romney does take some bruises in the early states, whether at the expense of Mr. Gingrich, Mr. Perry, Mr. Huntsman or even Mr. Paul. But then the other candidate runs out of steam. Mr. Romney recovers and wins, perhaps after a strong performance in Michigan on Feb. 28, on Super Tuesday.

Even candidates as strong as George W. Bush, who was in an absolutely dominant position in the Republican primary in 2000, normally lose a few states. The advantage of winning the prevoting phase of the nomination process known as the invisible primary is that you can give up a touchdown or two and then still come back to win the game in overtime.

But if expectations get too far ahead of themselves, Mr. Romney might be only one Howard Dean scream or one Ed Muskie teardrop from becoming genuinely vulnerable. Mr. Romney would be wise to ensure that he keeps them in check.