The most interesting scenario, however, is the case where the unpledged delegates would be sufficient to give a candidate a majority, but his claim to the nomination was somewhat tenuous. Suppose, for instance, that Mr. Romney had 43 percent of the delegates, Mr. Santorum 37 percent, and about 8 percent of delegates had not yet pledged to a candidate. Incidentally, if you assume that the Republican contest will continue to go back-and-forth and make some guesses about how each state will vote, you wind up with a lot of scenarios like this one.

If all of these unpledged delegates aligned themselves with Mr. Romney in this case, he could win on the first ballot. But whether they would do so is an open question.

If Mr. Romney’s plurality lead seemed to be built upon structural advantages in the delegate selection process rather than popular support — say, for instance, that Mr. Romney had the most delegates, but Mr. Santorum was 10 points ahead in national polls at he time — some delegates might conclude that it was not in the best interest of the party to give him a helping hand.

There has not been a case in the recent past when the candidate who was ahead in delegates once the last primaries were held trailed in popular support — even George McGovern led in national polls by the time states like Michigan and California voted in May and June 1972. Such a scenario seems plausible this year, however.