Though by now a Washington insider, Biden fits the profile of the kind of voter Obama struggled to attract in his nomination battle against Sen. Hillary Rodham Clinton. He is a Roman Catholic with working-class roots from hardscrabble industrial heartland.

He grew up in Scranton, Pa., and it could be his relatives that Obama had in mind when he made his now-famous comment about people whose bitterness over their economic plight leads them to cling to religion and guns. He is likely to spend much of his time over the next 2 1/2 months campaigning in the crucial industrial battlegrounds of Pennsylvania, Ohio and Michigan.

Biden's persona and political personality are in so many ways the antithesis of Obama's. Obama speaks with soaring and often inspirational rhetoric; Biden may be windy, but he is also direct, blunt and plain-spoken. Obama may seem aloof. Biden is always in your face. Obama offers high-altitude passion, Biden's is ground level. Who else but Biden might have introduced his wife to the American people as "drop-dead gorgeous"?

Republicans jumped on the choice of Biden, who has spent more than three decades in the Senate, as one that undermines Obama's change message. Obama advisers were on TV Saturday pointing out that Biden has never become a creature of Washington, noting that he rides the train home to Delaware every night.

On the central foreign policy issue of the campaign, Obama and Biden began in different places. Obama opposed the invasion of Iraq, while Biden supported it. Even as late as the spring of 2007, they were on opposite sides in a vote on funding the war, with Obama opposed and Biden — pointedly among Democratic senators seeking the nomination — in favor.

Republicans will look to exploit those differences and use them to undermine Obama's criticism of McCain's judgment on the war. But Biden also has been one of the most prominent and vocal critics of Bush administration policy in Iraq and has pushed for significant changes in policy for some years.

Biden proved during the primaries that he is a skilled debater — another attribute that recommended him to Obama. Voters ultimately do not make their November decisions on the basis of running mates, but the vice presidential debate will be one moment when the choice matters.

But the real debating Biden will be expected to do will be with McCain. As he made clear on Saturday, he and McCain are friends of long standing and there is mutual respect between them. But that did not prevent him from leveling a series of attacks against McCain as a creature of President Bush's foreign and domestic policies and a senator of questionable judgment.

"I've known John for 35 years," he said. "He served our country with extraordinary courage, and I know he wants to do right by America. But the harsh truth is ... you can't change America when you boast. And these are John's words, quote: 'The most important issues of our day, I've been totally in agreement and support of President Bush."'

At another point, he drew this contrast between Obama and McCain: "These times call for a total change in Washington's world view. These times require more than a good soldier. They require a wise leader. ... A leader who can deliver the change we need."

A Democrat who is a Biden admirer and who watched Saturday's performance said, "This is not going to be a reticent vice presidential candidate."