WASHINGTON — One talks too much. The other hasn't talked enough.

For voters, Thursday's vice presidential debate promises a transfixing match between the loquacious veteran Sen. Joe Biden and the still-underexposed Alaska Gov. Sarah Palin.

For the campaigns, the encounter in St. Louis represents a potential white-knuckle moment: The free-wheeling Biden vs. the tightly managed Palin in a test of knowledge, fluency and grace before millions of TV viewers.

Vice presidential candidates seldom decide elections; people vote for who's at the top of the ticket. But in a contest as close as this one between Republican John McCain and Democrat Barack Obama, a misstep could set back either campaign in the final weeks before Election Day.

What's more, both Biden, the Democrat, and Palin, the Republican, have become Obama's and McCain's ambassadors to independent voters, but each with different tasks: Biden to reassure them about Obama, and Palin to reassure them about herself.

And while the stakes may not be as high as they were in Friday's presidential debate, the running mates face more land mines than Obama and McCain did.

Since Palin roused the Republican Party at its national convention this month, she has been undergoing a crash course in foreign policy. Her struggles with some answers in interviews have been lampooned on "Saturday Night Live."

"She just has to show some rational basis to policy positions, some knowledge basis," said Sen. Sam Brownback, R-Kan., a former GOP presidential candidate and McCain supporter. "I don't think it is a high expectation level at all for her."

In short, Palin must aim for the kind of coherence that appeared to elude her when she tried to explain to CBS news anchor Katie Couric why being governor of Alaska gave her a certain understanding of Russia.

"As Putin rears his head and comes into the air space of the United States of America, where do they go? It's Alaska," she said. "It's just right over the border. It is from Alaska that we send those out to make sure that an eye is being kept on this very powerful nation, Russia, because they are right there, they are right next to our state."

Still, Biden, who twice has run unsuccessfully for president, perhaps faces the greater challenge. He has years of foreign policy experience and an affinity for extemporaneous speech that can cause him trouble. And he already has strayed from campaign talking points and mangled history during his own interview with Couric.

Discussing why President Bush should explain why it is necessary to help the financial industry, Biden said: "When the stock market crashed, Franklin D. Roosevelt got on the television and didn't just talk about the, you know, the princes of greed. He said, 'Look, here's what happened.' " (Herbert Hoover was president when the market crashed in 1929 and radio, not television, was the medium of the day.)