Wednesday night’s third presidential debate was probably John McCain’s last real chance to alter the dynamics of an election campaign that has been trending decisively against him for the last few weeks.

He didn’t do it. He might, in fact, have made an already difficult situation for him worse, by an ill-advised decision to repeat some of his recent character attacks on Barack Obama that polls have shown are doing his campaign no good at all.

Ahead of the debate Senator McCain’s advisers acknowledged that their candidate was trailing the Democrat and they promised that he would be feisty, in a last effort to sharpen the distinctions between the two candidates with less than three weeks to go to election day.

Nothing wrong with that. The Republican has little choice but to go after Senator Obama on a range of issues on which the Democrat is vulnerable - taxes; the budget; spending; trade policy -and to raise legitimate questions about Mr Obama’s experience and suitability for the biggest job in the world.

But the main question before the debate was whether Mr McCain would deploy some of the tactics he and Senator Palin, his running-mate, have used of late to highlight Senator Obama’s past associations with some dubious characters; most notably William Ayers.

Mr Ayers is a rabidly anti-capitalist, anti-establishment extremist who led bombing campaigns against US government targets in the 1960s. Senator Obama has a somewhat tenuous relationship with him. Their paths have crossed in ways that one would normally expect between a respectable and ambitious politician seeking state and national office and a radical ex-terrorist.

But no-one has ever really made the case that the connection was a strong one. Yet Senator McCain has spent much of the last couple of weeks ineffectually hammering away at this connection as proof that Senator Obama cannot be trusted.

On Wednesday night, in the most tense moment of all three debates so far, Senator McCain again raised Mr Ayers. Tellingly, he did so in a way that suggested even he didn’t really feel comfortable about it:

“I don’t care about an old washed up terrorist. But as Senator Clinton (in the Democratic primary campaign) said in her debates with you, we need to know the full extent of that relationship.”

As ever Mr Obama brushed aside the implied allegation with barely a blink.

The problem for Senator McCain is that voters remain unmoved by the Ayers connection, which they see as too distant to matter. Much worse, they can’t really understand why Senator McCain keeps bringing it up when the US economy is spiralling into its worst recession in at least 25 years.

A poll for CBS published on Wednesday before the debate found that two thirds of Americans think Senator McCain’s campaign has dwelled too much on negative attacks.

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