"If you don't understand what the cause is," he said, "it's virtually impossible to come up with a solution."

Then the conversation turned to Iraq, where the sons of both Palin and Biden are serving this year.

"It would be a travesty now if we quit now in Iraq," Palin said, charging that Obama's plan to withdraw troops is "a white flag of surrender."

Biden countered, "with all due respect, I didn't hear a plan." And he noted that both the Bush administration and the Iraqi government have moved toward embracing Obama's 16-month timeline for withdrawal from Iraq.

"The only odd man out here, the only one left out, is John McCain," he said.

He called the war a fundamental difference between the two tickets. "We will end this war," Biden said.

"John continues to tell us that the central war on terror is in Iraq," Biden said. "I promise you if an attack comes on the homeland it's going to come as our security services have said. It's going to come from al-Qaida planning in the hills of Afghanistan and Pakistan. That's where they live, that's where they are, that's where it will come from."

The vice presidential debate was a highly anticipated prize fight of politics because of Palin's wobbly performance in a series of recent interviews, most notably with Katie Couric, the anchor of the "CBS Evening News."

In answers mocked by the actress Tina Fey on two separate weeks of "Saturday Night Live," Palin gave a rambling and at times incoherent response when asked why Alaska's proximity to Russia enhanced her foreign policy credentials, as she had suggested in an earlier interview with Charles Gibson of ABC News. Palin also drew a blank when Couric asked her which Supreme Court decisions she disagreed with beyond Roe v. Wade.

The debate came at a difficult juncture for the McCain campaign as an array of polls nationally and in swing states showed that Obama had gained significant momentum in the race. Polls also show that a majority of voters believe that Palin is not experienced enough to be president if McCain, 72, reaches the White House but cannot serve out his term.

Biden went into the debate with disadvantages of his own, particularly his propensity for windiness and history of gaffes. In an effort to bring attention to some of Biden's most embarrassing foot-in-mouth moments, the McCain campaign released a Web advertisement on Thursday afternoon that showed Biden declaring that Sen. Hillary Rodham Clinton "might have been a better pick than me" as the vice presidential nominee, among other verbal faux pas.