Washington - US vice presidential candidates Sarah Palin and Joe Biden were preparing to take centre stage Thursday night in a highly anticipated debate barely one month before the November 4 general election.

The stakes are high for Biden and Palin to demonstrate both their own competence and to tout the best qualities of their running mates, as they face off for the only time in this election campaign.

Some 3,100 journalists have been accredited to cover the debate at Washington University in Saint Louis, Missouri, and the 90 minute event is set to be broadcast by all major US television networks.

The focus in the run-up to the debate had been squarely on Palin, a one-term Alaska governor who was catapulted onto the national scene only weeks ago after being picked as Republican nominee John McCain's running mate.

By contrast, Biden, 65, a two-time former presidential candidate and Delaware senator for 36 years, was a well-known quantity when he was tapped in August by Democratic nominee Barack Obama.

For Palin, 44, the debate was her most important chance yet to convince the public that she was ready for the top job in the event something should happen to McCain, who at 72 would be the oldest president ever elected to a first term.

According to a Washington Post-ABC News poll, only 35 per cent believed she had the experience necessary to be president, down from 47 per cent in early September. By comparison, 70 per cent believed Biden has the necessary experience.

For Biden, long one of the party's top minds on foreign policy as chairman of the Senate's Foreign Relations Committee, the pitfalls of Thursday's debate were related more to over exposure.

Biden has a long reputation for making political gaffes on the campaign trail. The McCain campaign released a web video Thursday citing his biggest mistakes to date, including a comment that you could not walk into a US neighbourhood convenience store "unless you have a slight Indian accent."

Biden's most famous misstep dates back to 1987, when his first presidential candidacy collapsed after it was discovered that he had plagiarized parts of a speech by a British politician.

Yet despite being protected from the media by the McCain campaign, Palin's own gaffes in three recent interviews had provided much fodder for late-night comedians.

Her comments citing Alaska's proximity to Russia as foreign policy experience or confused answers on a 700-billion-dollar financial rescue plan have led some conservative commentators - who are supposed to be on her side - to publicly ask that Palin bow out of the race.

But aside from watching their words, both candidates were expected to square their own political perspectives with those of their running mates.

Biden will have to answer critics that argue his 36 years in Washington do not sit well with the 47-year-old Obama's message of change. Palin must answer for some conservative beliefs, such as a skepticism of the man-made causes of global warming, that do not square with McCain's own views.

The other factor hovering over the debate was that of gender. Palin would be the first ever female vice president, and Biden will have to avoid adopting "a posture of being aggressive toward her or, worse, condescending," said Bob Shrum, who helped John Kerry prepare for his 2004 presidential debates, in an interview to Newsweek.

Palin should exploit her more personal side, put herself forward as "a likable, caring person who believes what she says," said Stuart Stevens, who helped Republican Vice President Dick Cheney prepare for debates with Democrats Joe Lieberman and John Edwards in 2000 and 2004 respectively.

Given that dynamic, the debate may have its biggest impact on one- time supporters of Hillary Clinton, who lost the Democratic primary to Obama and was then passed over as his pick for vice president.

Biden, in one of his most famous gaffes of the current campaign, admitted last month that Clinton "might have been a better pick than me." (dpa)