Barack Obama and John McCain enter potentially treacherous waters tonight when they take questions from an audience of swing voters in their second presidential debate - one of the Republican’s last chances to change the trajectory of a race that has shifted towards his rival.

Mr McCain has spent the past three days with advisers in his home state of Arizona, putting in far more hours of preparation than before the first debate in Mississippi 12 days ago, and weighing up how aggressively to go after Mr Obama.

All the signs are that Mr McCain will try to convince voters that Mr Obama is too reckless a bet for the White House in such turbulent times at home and abroad, as he tries to change the dynamic of the race with less then a month until the election.

The Republican has got a big wish in tonight’s encounter at Belmont University in Nashville: it will be a town hall-style forum, with the two candidates sitting on director’s chairs and answering questions directly from an audience of Americans. It is a medium in which the Republican thrives and in which he feels the most comfortable.

Yet the format has a degree of unpredictability and potential danger for the candidates - a third of the questions are also coming via the internet - as voters can spring surprises, and often think in different ways from the rota of veteran Washington journalists who usually moderate the debates.

Tom Brokaw, of NBC, will select the questions. Questioners cannot follow up - nor can Mr Brokaw - and the rules will greatly limit interaction between the candidates.

Mr McCain’s greatest successes as a candidate have come through the town hall format, and his back-from-the-dead comeback during the Republican primary this year was built on an exhausting schedule of such encounters in New Hampshire.

He has repeatedly challenged Mr Obama to a series of ten town hall encounters across the country. Mr Obama has refused, saying that the three formal presidential debates are sufficient. The Republican has blamed Mr Obama’s failure to agree to a series of meetings as one of the reasons the campaign has become so negative.

The Obama campaign has been raising expectations for Mr McCain with an absurd level of hyperbole. One aide to the Democratic candidate described the Republican as the “Michael Phelps of townhall performers” - a reference to the record-break-ing Olympic swimmer. It was perhaps an unfortunate analogy because Mr McCain cannot raise his arms above shoulder height because of wounds that he suffered in Vietnam.

The debate is expected to focus on the economy, a now dominant voter issue that favours Mr Obama and has been responsible for his recent momentum. Yet Mr McCain believes that he can make ground by painting Mr Obama as a tax-and-spend liberal.

There is still significant resistance to Mr Obama among undecided voters because of his inexperience, and Mr McCain will also try to play up his lack of legislative achievements.

Mr Obama, who has been preparing in Asheville, North Carolina, will strive to keep the debate focused on the economic crisis. Despite his recent gains over Mr McCain, he is far from assured of victory on November 4.

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