Washington - Sarah Palin, the running mate of Republican White House hopeful John McCain, has erased some of the doubts over her fitness for the top job after a fast-paced and wide-reaching debate with Joe Biden, vice presidential pick of Democrat Barack Obama.

Coming into Thursday night's debate, Palin, a one-term governor of Alaska, had faced limited media interviews and exchanges since being thrust into the national limelight just five weeks ago.

The three national interviews she did offer had been derided by critics as evidence that she was unprepared to be a heartbeat away from the presidency. The initial excitement among conservatives over her pick by McCain had eased, as even some right-wing commentators suggested it was time for her to leave the stage.

The tone changed dramatically after Thursday night's one-and-only vice presidential debate in St Louis, Missouri.

Palin appealed to middle-class Americans with a plain-spoken style and appeared to have held her own against Biden, a US senator for 36 years and master of foreign policy.

"Sarah Palin was threatening to become an embarrassment for the McCain ticket and she was threatening to drag it down," said David Gergen, an advisor to four former presidents and CNN analyst. "I think she erased that tonight."

While early opinion polls suggested Biden had performed better overall, winning had never been the key requirement for Palin.

Instead, she needed to demonstrate competence on national and international issues to ease the concerns of voters who believed she was unqualified for the job of vice president.

Palin challenged Biden on issues of foreign policy, the economy, taxes, health care and energy during the widest and longest test she has had before the American public since being tapped as vice president in late August.

She touted McCain and herself as reformers who recognized the problems of the middle class and had stood up against their own party - the ticket's key message given the 30-per-cent approval ratings of President George W Bush.

"I think we need a bit of reality from Main Street Wasilla brought to Washington," Palin said, referring to her Alaskan home town where she was mayor before being elected governor in 2006.

While nearly all eyes were on Palin before and after the debate, Biden answered his own critics who had pointed to the Delaware senator's tendency for political gaffes as well as making long, drawn-out arguments.

Biden touted his own working-class roots growing up in Pennsylvania. He slammed McCain for being "out of touch" and siding with Bush on the major issues of the election.

"A maverick he is not on the important, critical issues that affect people at the kitchen table," Biden said.

Neither candidate made any serious mistakes. An opinion poll by CNN showed 84 per cent of viewers thought Palin had performed better than expected, while 64 per cent said Biden also beat expectations.

The debate came as election momentum was swinging in Obama's favour just one month before the November 4 presidential poll. The change has been largely due to the financial crisis, which has shifted focus back to economics - Obama's stronger suit - and away from McCain's strength of national security.

Vice presidents are rarely a decisive factor in the choices of US voters but there had been a perception that a bad performance by Palin could seriously hurt the McCain campaign.

McCain, 72, took what was widely considered a massive political gamble by choosing Palin. Her conservative social views had energized a Republican base that had been skeptical of McCain's maverick reputation and historically uneasy relationship with Republicans.

But Palin's lack of experience left many voters doubting whether she could handle the top job in the event something should happen to McCain, who would be the oldest president ever elected to a first term in office.

On Thursday, even opponents were forced to admit that Palin could handle the tough national limelight.

"I think she showed she is certainly capable of going toe to toe with (Biden) who is more than qualified to be vice president, if not president of the United States," said Geraldine Ferraro, a Democrat and the only other female vice presidential candidate in US history. (dpa)