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“Mark me down as one who says not being around a long time ain’t a bad thing, because, as I said to you before, there has to be change, there has to be a disruption. You know I’m a disrupter,” she said.

Mr. Biden, who represents Delaware, also brings blue-collar credentials to a campaign that has faltered recently under charges that Mr. Obama is an elitist celebrity. And Mr. Biden brings a willingness and the personal knowledge to do the dirty work required of a vice-presidential nominee in attacking the other party’s nominee.

Speaking in Springfield, Ill., Mr. Biden said he knows Washington is broken and said Mr. Obama is the man to fix it.

David Sirota, a Democratic strategist who has been pushing the Democratic Party to embrace more liberal policies, said as a pick, Mr. Biden was “a shade on the good side of mediocre.”

More worrisome, Mr. Sirota said, was Mr. Obama’s willingness to tap someone who has been a friend to big-business interests such as banks that issue credit cards and who voted to authorize the Iraq war. Mr. Sirota said the selection “gives us some disturbing clues about the Illinois senator’s attitude toward the economic progressive movement and the antiwar movement.”

A two-time presidential candidate in his own right, Mr. Biden has been at the center of some of the biggest Washington fights. He claims credit for helping keep Judge Robert Bork off the Supreme Court, and he was chairman of the Foreign Relations Committee in 2002 when the Senate voted to authorize the war in Iraq.

But Mr. Biden’s plain-spoken directness brings with it a reputation for gaffes.

He withdrew from his first presidential bid, for the 1988 Democratic nomination, after he acknowledged using words from a British politician without credit in one of his speeches. His 2008 bid got off to a bad start when he stepped on his own announcement, calling Mr. Obama “the first mainstream African-American who is articulate and bright and clean and a nice-looking guy.”

The Republicans lost no time Saturday in setting up a “Biden Gaffe Clock.”

Like Mr. McCain, Mr. Biden’s years in the Senate also have left him on all sides of some of the major issues. He voted for the North American Free Trade Agreement and for letting China into the World Trade Organization, but has since opposed both fast-track trade authority and the Central American Free Trade Agreement.

That reversal was enough on Saturday to earn the Obama-Biden ticket the label of “the most pro-working-family presidential ticket in a generation” from the Service Employees International Union.

And Mr. Biden was a supporter of the Iraq war, even going so far as to say it was obvious then-Iraqi strongman Saddam Hussein was concealing weapons of mass destruction, but later became a critic of the war, proposing to divide the country into three parts.