Of the 11 competitive caucuses since 1980, the candidate leading in the polls a month in advance won them 8 times. That’s a pretty good batting average for Mr. Gingrich. As difficult as it is to poll the caucuses, surveys of Iowa have tended to be pretty reliable in the past.

It is important to note, however, that Mr. Gingrich’s lead is smaller than that of most candidates from past years. In the five prior cases where a candidate had a single-digit lead, he won twice but lost three times. The losses came in 1980, when Ronald Reagan lost to a better-organized candidate in George H.W. Bush, in 1988, when there was an essential four-way tie in the Democratic caucuses and the nominal leader, Gary Hart, lost to Richard Gephardt, and in 2004, when Howard Dean’s support collapsed and he lost to John Kerry.

Perhaps Mr. Romney can take solace from the comeback of Mr. Kerry, a candidate to whom he is sometimes compared.