People like to cite counterexamples: The Republicans nominated Barry Goldwater, after all, and the Democrats nominated George McGovern. But Goldwater and McGovern, for all their weaknesses, were far more credible nominees than a Perry, a Herman Cain, a Michele Bachmann, a Newt Gingrich. They were too extreme to win the general election, but they were not political novices or washed-up self-promoters, and they had a mix of eloquence and experience that’s largely absent from the current Republican field. (Watch clips of Goldwater being interviewed by William F. Buckley Jr. on “Firing Line,” and then try to imagine how Perry would fare in the same format.)

Everyone has an incentive to play down these realities and play up Romney’s vulnerabilities instead. The press needs the illusion of a hard-fought campaign to keep its audience from straying. Democrats enjoy the spectacle of right-wing infighting, and benefit politically from it as well. And Republicans don’t want to admit that America’s conservative party is destined to nominate a politician who embodies the very tendencies that the conservative movement came into being to resist: technocracy, ideological flexibility, Northeastern moderation.

It is a rather extraordinary turn of events. But when you have eliminated the impossible, as Sherlock Holmes told Watson, then whatever remains, however improbable, must be the truth. This rule holds for presidential contests as well as for whodunits: Romney is improbable, but his rivals are impossible, and so he will be the nominee.