Leading article: Now the Republicans must choose

Sarah Palin's announcement that she will not seek the presidency, following the similar decision from New Jersey Governor Chris Christie 24 hours earlier, appears to have settled the Republican field for 2012.

A late entrant cannot entirely be ruled out. But to all intents and purposes, this is Mitt Romney's moment. The most important question in American politics now is: can the unloved frontrunner seize it?

Ms Palin's non-candidacy came as no surprise. Not only did a sizeable majority of Republicans simply not want her in the race. It has also long been clear she lacked the seriousness, self-discipline, and raw desire needed to survive the gruelling ordeal of a primary campaign. But up to the last, the mere possibility she might run underlined the central truth of the contest: that for all Mr Romney's credentials, experience and preparation, Republican voters have yet to warm to him.

Every opponent whose sudden surge has made the headlines – first Michele Bachmann, then Rick Perry, then Mr Christie, and now Herman Cain – has been framed as the anti-Romney candidate. With all possible alternatives finally exhausted, the Republicans must now choose. Assuming the vogue for Mr Cain does not last, that choice boils down to Mr Romney or Mr Perry. Given President Obama's sinking fortunes, it is not impossible one or other will reach the Oval Office.

For Republicans it is a matter of heart vs head. If Mr Perry can recover from recent stumbles, and define the primary contest in ideological terms, he will prosper. Mr Romney, by contrast, has sought to make the election exclusively about jobs and the economy, relying on his proven competence as a businessman and then as Governor of Massachusetts. There is little doubt that in a general election, where centrists and independents decide, he would be the most formidable opponent for Mr Obama. Over the next six months, he must persuade his party of that truth.