Mrs Palin's return on Wednesday marked her first solo campaign foray after spending the last week in tightly choreographed appearances with the Republican presidential contender, Senator John McCain. Seeking to recapture some campaign-trail sizzle, the Democrat candidate, Senator Barack Obama, has revealed that former president Bill Clinton will hit the hustings for him during the weeks leading up to election day.

"There's nobody smarter in politics," Senator Obama said during an appearance on the Late Show With David Letterman on Wednesday night. "And he is going to be campaigning for us over the next eight weeks, which I'm thrilled by."

A spokesman for the former president, Matt McKenna, said Mr Clinton would campaign for Senator Obama at a yet-to-be announced site in Florida on September 29, with plans for more fund-raising and events in the works. Senator Hillary Clinton has already been campaigning for Senator Obama.

Aside from an interview with People magazine on the day Senator McCain introduced her as his vice-presidential choice, Mrs Palin has been off-limits to media questions, although Britain's Daily Telegraph did manage to hunt out an image of her from a photographic session she did with the US Vogue earlier this year and put it on the front page. Her return to Alaska is part of a carefully orchestrated introduction to the media.

Since Mrs Palin entered the campaign, she has become the centre of attention, mobilising the conservative Republican base and drawing crowds of adoring fans who chant her name.

On Wednesday there were more ugly stumbles for the Obama campaign. The chairwoman of the South Carolina Democratic Party, Carol Fowler, told the Politico website that Senator McCain had chosen a running mate "whose primary qualification seems to be that she hasn't had an abortion".

Mrs Palin opposes abortion and gave birth to her fifth child, Trig, earlier this year after finding out during her pregnancy the baby had Down syndrome.

And at a town hall in Nashua, New Hampshire, the Democratic vice-presidential nominee, Joe Biden, praised Hillary Clinton so much that he appeared to say she would be a better vice-president than he would be.

"Let's get that straight," Senator Biden said. "She's a truly close personal friend; she is qualified to be president of the United States of America. She's easily qualified to be vice-president of the United States of America and quite frankly it might have been a better pick than me, but she is first-rate."

Senator Obama spent most of the day on the defensive over his comment on Tuesday that "if you put lipstick on a pig, it's still a pig".

He responded to a Republican attack on his words by saying: "Enough is enough. Spare me the phony outrage, and the talk about change. People are looking for answers on the economy, on health care, on education."

No love has been lost between Senator Obama and Mr Clinton in the past. During the Democratic presidential primaries, the former president acted as his wife's chief defender from Senator Obama's challenge to her candidacy.

Mr Clinton portrayed Senator Obama as too inexperienced to stand for president, and Senator Obama said it was sometimes hard to tell which Clinton he was standing against. The two did not speak for nearly a month after the campaign ended. But the pair appear to have patched up their differences and the Democrats are desperately in need of some good news.

The latest polls will have the Democrats worried. The Gallup daily tracking poll has Senator McCain leading Senator Obama 48 per cent to 43 per cent among registered voters..

New CNN/ Time polls in four states show Senator Obama and Senator McCain are each leading in the two states the presidential nominee from their party won in 2004. Senator Obama leads in Michigan and New Hampshire, while Senator McCain leads in Missouri and Virginia. The Democrats have high hopes of winning Virginia, which has voted Republican for 40 years, after they registered more than 200,000 new voters in the primaries. But the test will be in Ohio and Pennsylvania, where both sides have been furiously campaigning.

More than 15,000 people turned out to hear Senator McCain and Mrs Palin speak at a rally in a park in Fairfax, Virginia, a suburban area that is part of Washington DC. White women in their 30s, 40s, 50s and 60s made up a large part of the crowd. "Sarah Palin is the most remarkable success story in the history of US politics," the former senator and actor Fred Thompson said to wild applause.She was mostly greeted enthusiastically, though her speech was her standard one from the convention. But there were a few flat moments. She said that with the help of those present she would go to Washington to shake things up - apparently oblivious that Fairfax county is effectively a suburb of Washington. There also seemed to be slightly less enthusiasm for her line about driving out the special interests - after all, northern Virginia is where many lobbyists live and work.

- with agencies