The two US vice-presidential candidates are set to butt heads  for the first and only time in a TV debate.

Democrat Joe Biden and Republican Sarah Palin will front a studio audience at Washington University in St Louis, Missouri in a matter of hours.

For Governor Palin in particular, the debate is seen as key, after her poll ratings have fallen amid voters' concern about her relative inexperience. A new poll by the Washington Post suggests that 60% of voters now see Mrs Palin as lacking the experience to be an effective president. This then affects the vote for Republican Presidential hopeful, Senator John McCain.

Of course, Senator Biden could also be vulnerable should he make a mistake or misjudgement in his responses.

Mrs Palin has spent the week sequestered at Mr McCain's ranch in Arizona as she prepares for the debate, which will be moderated by PBS journalist Gwen Ifill.

Meanwhile, Mr Biden has been preparing with Democratic governor Jennifer Granholm of Michigan.

Tags: barack obama, Joe Biden, john mccain, politics, sarah palin, US Election