"She'll be just fine. She'll do fine tonight," McCain told MSNBC, rejecting criticism that his campaign had mishandled her by shielding her from the media.
"She's experienced. She's knowledgeable. She's very strong person. I'm proud of her record, and I'm proud of her."
McCain's campaign, which has stalled in the wake of the global financial crisis, said on Thursday he had conceded the battleground state of Michigan in order to focus on states where he has a better chance of beating Barack Obama, the Democratic candidate.
"It's been the worst state of all the states that are in play and it's an obvious one from my perspective to come off the list," said Greg Strimple, a senior McCain advisor.
John Nichols, a political writer at The Nation magazine, told Al Jazeera that the challenge for Palin was to find an issue to focus on to revive the Republican campaign.
Watching for mistakes
However, concerns over Palin's experience have increased since she gave a series of interviews on foreign policy, the economy and the US supreme court.
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She has faced ridicule for some of her answers, including citing Alaska's proximity to Canada and Russia as valid foreign policy experience.
However, Al Jazeera's Rob Reynolds in St Louis said it would be a mistake to write Palin off as she had done well during debates for the governorship in Alaska.
Biden, 65, the chairman of the senate foreign relations committee, faces his own set of challenges as he tries to rein in what analysts have said is his tendency to make mistakes and talk too much.
The McCain campaign has launched an online advertisement mocking Biden, highlighting his comments about his own high IQ, the high number of foreign workers in convenience stores and Hillary Clinton's better suitability for his vice-presidential slot.
"Ready to gaffe? Yes. Ready to lead? No," the advertisement says.
Palin and Biden have spent several days off the campaign trail to prepare.
Palin has been at McCain's Arizona retreat and Biden at home in Delaware, although he returned to the senate in Washington to vote for the government's bailout package on Wednesday.
The Biden-Palin encounter follows last week's first presidential debate between Barack Obama and McCain in Mississippi.
Opinion polls judged Obama the winner of that contest, and the Illinois senator has since maintained his lead in national polls and in key battleground states.
The television audience could be the largest for a vice-presidential debate, surpassing the nearly 57 million who watched in 1984 when then vice-president George Bush, the current president's father, debated with Geraldine Ferraro, the first woman vice-presidential nominee for a major party.