Mr. Biden is Roman Catholic, giving him appeal to that important voting bloc, though he favors abortion rights. He was born in a working-class family in Scranton, Pa., a swing state where he remains well-known. Mr. Biden is up for re-election to the Senate this year and he would presumably run simultaneously for both seats.

Mr. Biden is known for being both talkative and prone to making the kind of statements that get him in trouble. In 2007, when he was competing for Mr. Obama for the presidential nomination, he declared that Mr. Obama was “not yet ready” for the presidency.

The McCain campaign jumped on that early Saturday, as it responded to the selection, offering a glimpse into the line of criticism that awaits the Democratic ticket.

“There has been no harsher critic of Barack Obama’s lack of experience than Joe Biden. Biden has denounced Barack Obama’s poor foreign policy judgment and has strongly argued in his own words what Americans are quickly realizing  that Barack Obama is not ready to be President,” said Ben Porritt, a spokesman for Mr. McCain.

Although Mr. Biden is not exactly a household name, he is probably the best known of all the Democrats who were in contention for the spot, given his political and personal history (not to mention his regular appearances on the Sunday morning television news shows). He first ran for the Senate from Delaware when he was just 29.

Mr. Biden has run twice for the presidency himself, in 1988 and again in 2008, dropping out early in both cases. He was also the chairman of the Senate Judiciary Committee during two of the most contentious Supreme Court nomination battles of the past 50 years: the confirmation proceedings for Robert H. Bork, who was defeated, and Clarence Thomas, who was confirmed after an explosive hearing in which Anita Hill had accused Mr. Thomas of sexual harassment. Mr. Biden led the opposition to both nominations, although he came under criticism from some feminists for not immediately disclosing what were at first Ms. Hill’s closed-door accusations against Mr. Thomas.