The big question before Thursday’s night’s eagerly awaited vice-presidential debate between Sarah Palin and Joe Biden was which Sarah Palin would show up? The populist, wow-em, conservative-delighting flame-thrower from the Republican convention four weeks ago? Or the mumbling, incoherent clearly-out-of-her-depth ingénue from recent television interviews?

Thanks to those disastrous TV outings, expectations for her performance before the debate were so low that she could have exceeded them simply by coming on stage and smiling beatifically for an hour and a half.

But she did much better than that, once again confounding her condescending critics with a pleasing fluency and an engaging folksiness, and for the moment at least putting a crucial floor under the McCain campaign’s freefall of the last two weeks.

It was in fact, an impressive debate. Expectations for Senator Biden had not been all that high either, given his long-established tendency towards prolixity and logorrhea. But he delivered a highly effective performance. He was coherent and impassioned and mercifully concise, a 36-year veteran of the US senate clearly in command of his political brief. He also skilfully avoided the pitfall of appearing unchivalrous by going after his opponent. He scored points instead by sticking resolutely to repeated, fierce attacks on her running-mate (and his Senate colleague and friend) John McCain.

On the substance, you might choose to award the debate – just - to Senator Biden. He seemed more in command of the issues and answered the questions from Gwen Ifill, the moderator, more directly, while Mrs Palin – poised and fluent as she was - gave the impression at times she was cleaving closely to campaign talking points.

But impressions may matter more to voters than evidence of detailed knowledge of Washington policymaking.

The Alaska governor set the tone for the evening by greeting Senator Biden on stage at the start with a disarming question, clearly audible through her lapel microphone, “May I call you Joe?”

She punctuated most of her answers with the sort of colloquialisms rarely heard in the liberal metropolitan citadels of New York, Washington and Los Angeles.

“Doggone it”, “You betcha!”, she said for emphasis at times. She talked at one point about the values of “Main Street, Wasilla”, the town where she was mayor until a few years ago. She sang the praises of elementary school pupils and made a virtue repeatedly of not knowing much about the ways of Washington.

The folksiness helped reinforce the message of the Washington outsider and the McCain campaign’s theme of reform of a corrupt and out-of-touch government. Neither she nor Sen Biden had much new to say on the financial crisis engulfing America but her insistence that at root the problem was not just about Wall Street corruption but about a failure of personal responsibility among all Americans probably resonated with voters.

Mrs Palin seemed more shaky than Senator Biden on foreign policy, as you might expect. She misidentified Gen David McKiernan, the Nato commander in Afghanistan, calling him Gen McLellan, and seemed to misunderstand a question about possible US use of nuclear weapons.

Senator Biden’s most effective moment was probably when he challenged Mrs Palin’s claim that Senator McCain was a maverick, who had been out of step with President George Bush. The Democratic candidate reeled off a long list of foreign policy issues – from Iraq to Iran and Israel – where Senator McCain had dutifully supported President Bush.

How much the vice-presidential debate matters is highly questionable. Perhaps the single most famous moment in any such debate was when Lloyd Bentsen, the Democratic vice-presidential nominee in 1988 eviscerated Dan Quayle, his Republican opponent in 1988 by deriding his claim to the John F Kennedy mantle. A month or so later Senator Bentsen and his running-mate Michael Dukakis, went on to lose that election in a landslide to George Bush Senior.

But in this election, at a critical moment when the tide has clearly turned against Senator McCain, a dreadful performance by Mrs Palin, reminiscent of her recent TV appearances, might have come close to burying the Republicans.

Instead, she seemed sure-footed and likeable, erasing many of the doubts that surrounded her and breathing at least a little hope to a fairly demoralised Republican camp.

Last week, Senator McCain probably lost his first debate against Senator Obama by not winning it.

On Thursday night Mrs Palin won her debate by not losing it.

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