Rarely has a vice-presidential showdown been packed with such political importance. Ms. Palin’s unsteady performances in recent interviews turned this debate into can’t-miss television, but they have also raised questions  from conservatives, among others  about the soundness of Mr. McCain’s judgment in picking a relative newcomer as his running mate. Recent polls have suggested that his shifting statements on the economic bailout talks in Washington have not reassured some of these conservatives, raising the stakes for Ms. Palin to deliver steady, informed answers and repartee in the debate.

Mr. Biden’s aides had their own concerns before the debate, worrying that a single gaffe by him could shift the onus off Ms. Palin. They worried that even the slightest miscalibration of his tone, body language and mien could imply condescension or worse toward Ms. Palin and become the story of the night.

With both candidates keeping their cool and addressing each other politely with honorifics  Mr. Biden said “Sarah Palin” at one point and then correct himself with “Governor Palin”  there was a certain symmetry to the debate.

Both candidates have a son preparing to serve in Iraq. Every time Mr. Biden seemed to criticize Mr. Bush, Ms. Palin would mention “mavericks.” And when Mr. Biden criticized the Bush administration at one point, Ms. Palin replied: “Say it ain’t so, Joe. There you go again, pointing backwards again.”

The two candidates have both faced personal challenges, too: Ms. Palin’s baby son has Down syndrome, while Mr. Biden, in the 1970s, lost his wife and daughter in a car accident. As he recalled that time and the near-death of one of his sons, Mr. Biden briefly choked up  the one moment of raw emotion in an otherwise stable debate between two fairly disciplined candidates.

“The notion that, somehow, because I’m a man, I don’t know what it’s like to raise two kids alone, I don’t know what it’s like to have a child you’re not sure is going to make it,” Mr. Biden said. “I understand as well as, with all due respect, the governor or anybody else, what it’s like for those people sitting around that kitchen table. And guess what? They’re looking for help.”