In other words, the race comes down to Romney, Gingrich, and Paul. Romney and Gingrich are the main contenders, but Paul wins about 10 percent of the vote in the primaries and 20 percent in the caucuses, where his dedicated followers are especially effective. (There’s some historical precedent to this speculation: In 2008, Paul won 17 percent of the vote in the Alaska caucuses and 18 percent of the vote in the Maine caucuses.) Unlike in 2008, however, Paul actually wins some delegates, because GOP rules now mandate that states that hold primaries before April distribute their delegates on a proportional basis. Moreover, the especially early states, including even New Hampshire, lose half their delegates as a penalty for holding their elections before February.

Romney wins northern states, such as Vermont, and Gingrich wins southern states, such as Tennessee. Conservatives in states Romney won in 2008 abandon him for Gingrich, decreasing his victory margin. (Let’s say he wins the Alaska caucuses, but only by 41 percent, for instance.) After April 1, when the winner-take-all primaries begin, Romney cleans up: He wins all the votes of big blue states such as New York, California, and New Jersey. But Gingrich holds his own, scoring strong wins in Pennsylvania and Ohio. When the delegates assemble in Tampa, no candidate has a majority.