It’s been a long time since we had a brokered convention. But if the nomination were stalemated after the first dozen or two states had voted, or if it seemed as if an “unelectable” candidate were in danger of being nominated, a candidate like Mr. Christie could potentially enter the race very late, demonstrating his popular support by performing strongly in late-voting states like California, Ohio and North Carolina and then winning the nomination as a consensus choice on the floor of the convention.

These scenarios, as fun as they would be for campaign reporters, are not at all likely. Probably, the beneficiary of all the difficulties in the conservative half of the Republican electorate will be Mr. Romney. But if the Republican electorate just does not take to Mr. Romney once the voting begins, the party will still have to find a nominee somehow.

That’s why I reserve an outside chance — I’d ballpark it at about 25-to-1 against — that the candidate to win the Republican nomination will be one who is not running now. Somewhat contrary to the common perception, the Republican calendar is quite backloaded this year — roughly as many delegates will be at stake in April, May and June as in January, February and March. So if the party does not find its nominee early, some back-door possibilities may open up.