Biden responded that John McCain, too, had voted against funding, and said the Republican presidential candidate had been "dead wrong on the fundamental issues relating to the conduct of the war."
The clash over Iraq was the most personal, and pointed, of the 90-minute debate in which Palin repeatedly cast herself as a non-Washington politician, part of a "team of mavericks" that she said was ready to bring change to a country demanding it.
From the opening moments of the debate, Biden sought to make McCain out as a straight-ahead successor to an unpopular President Bush. "He voted four out of five times for George Bush's budget, which put us a half-trillion dollars in debt and over $4 trillion in debt since he got here," he said of McCain.
Palin merely accused Biden of reciting the past rather than looking to the future. "Americans are cravin' that straight talk" that McCain offers, she said.
The two running mates debated for 90 minutes on a stage at Washington University, their only encounter of a campaign with little more than one month to go.
Recent polls show Obama with a small but perceptible lead, and Republican officials said earlier in the day that McCain had decided to pull out of Michigan, conceding the state to the Democrats. At the same time, his own aides said the campaign may soon begin to advertise in Indiana -- a state that has voted Republican in every presidential election since 1968.
Palin faced enormous challenges of her own, though. After five weeks as McCain's ticket-mate, her poll ratings had begun dropping and even some conservatives have questioned her readiness for high public office.
Her solo campaign events are few, and she has drawn ridicule for some of her answers in the few interviews she has granted -- including her claim that Alaska's proximity to Russia gives her an insight into foreign policy.
After intense preparation -- including two days at McCain's home in Sedona, Ariz., there was only one obvious stumble, when she twice referred to the top U.S. commander in Afghanistan as "Gen. McClellan." His name is David McKiernan.
Biden's burden was not nearly as fundamental. Although he has long had a reputation for long-windedness, he is a veteran of more than 35 years in the Senate, with a strong knowledge of foreign policy as well as domestic issues.
For much of the evening, the debate unfolded in traditional vice presidential fashion -- the running mates praising their own presidential candidate and denigrating the other.
Palin said Obama had voted to raise taxes 94 times -- an allegation that Biden disputed and then countered. By the same reckoning, he said, McCain voted "477 times to raise taxes."
They clashed over energy policy, as well, when Palin said Obama's vote for a Bush administration-backed bill granted breaks to the oil industry. By contrast, she said that as governor, she had stood up to the same industry, and noted that McCain had voted against the bill Obama supported.
Biden said that in the past decade, McCain had voted "20 times against funding alternative energy sources and thinks, I guess, the only answer is drill, drill, drill."
"The chant is, `drill, baby drill," Palin countered quickly, unwilling to yield to Biden on that issue -- or any other.
On the environment, Palin declined to attribute the cause of climate change to man-made activities alone. "There is something to be said also for man's activities, but also for the cyclical temperature changes on our planet," she said, adding that she didn't want to argue about the causes.
Biden said the cause was clearly man-made, and added, "If you don't understand what the cause is, it's virtually impossible to come up with a solution."