Gavin Hewitt

Nashville, Tennessee: Here's a question: if your opponent gets dirty, do you join him in the mud?

It came up in the media centre, or "Spin Alley", in the hours before the debate in Nashville.

Robert Gibbs, an Obama strategist, was chewing the fat with a few reporters. Spin, these days, is a 24-hour business. It precedes a debate and goes into over-drive afterwards.

Gibbs was wondering out loud which McCain would show up tonight. Would he be angry and desperate, or the candidate who thrives on town-hall meetings?

And then Gibbs was asked about going negative. The Obama camp believes we are entering a new phase in the campaign. The gloves have started to come off and the Republicans want to make the election a referendum on Obama the man.

They regard Sarah Palin's comment that Obama was "palling around with terrorists" as straight out of the nasty bag. So you might think that with Obama pulling ahead in some of the key states, his campaign would stay out of the street fights. But Gibbs saw it very differently: "If they want to get dirty - we'll go there with them."

Barack Obama is of the same mind: "We don't throw the first punch but we'll throw the last."

So, when the Republicans raised Obama's links to the former radical Bill Ayres, the Democrats hit back by high-lighting McCain's links to Charles Keating and the savings and loans scandal.

So why not stay out of a "slug-fest"? Imprinted in the Democrats' DNA is what happened to John Kerry and the attacks on his time in Vietnam in the Swift boats.

While his opponents questioned Kerry's war-time story and his heroism, his campaign said little. That enabled the story to take root. The Obama campaign believes not only in instant rebuttal but fighting back.

It's an interesting strategy because, as the ailing economy has worked for Obama, so he has tried to be reassuring and presidential. That has so irritated some Republicans that they've accused him of "coasting" to the White House. So, they would like nothing better than to draw him into a tough fight.