DENVER -- Sen. Barack Obama became the Democratic Party's nominee for president last night. He celebrated the milestone with a surprise visit to his party's convention to congratulate his new running mate, Sen. Joseph R. Biden Jr., and the extended Biden family.

As the new team basked in the delegates' applause just after the vice presidential nominee addressed the convention, generations of Bidens, from the senator's mother to the nominee's grandchildren joined them on the Pepsi Center stage.

Mr. Obama's nomination by acclamation, on the motion of longtime rival Sen. Hillary Rodham Clinton, brought him a place in history as the first African-American standard-bearer of a major political party.

In his prime-time address, Mr. Biden fought to propel the Democratic ticket past a still higher historic threshold with a sometimes stinging indictment of Sen. John McCain, the man who will accept nomination as the Republican nominee next week in Minnesota.

The Scranton native's sharp attack on Mr. McCain, which was a centerpiece of his address, contrasted with the warmth of the family display on the podium.

"These times require more than a good soldier," Mr. Biden said. "They require a wise leader, a leader who can deliver change, the change everybody knows we need. Barack Obama will deliver that change."

Mr. Obama's unannounced appearance, timed perfectly to catch the end of the prime time television audience on the East Coast, gave him a chance to reinforce the convention's search for unity. He enthused that Sen. Hillary Rodham Clinton had "rocked the house," with her speech Tuesday. And he praised former President Bill Clinton's speech earlier in the evening, saying that it had "reminded us of what it's like when you have a president who actually puts people first."

The abbreviated roll call that certified Mr. Obama as the leader of America's oldest political party was one more attempt to exorcise the tensions of the long primary struggle between the Democratic leaders. The traditional litany of the states moved along normally until it reached New York. The Pepsi Center filled with cheers as Mrs. Clinton appeared on the floor.

Leaning into the microphone under her state's standard, she said, "On behalf of the great state of New York, with appreciation for the spirit and dedication of all who are gathered here, with eyes firmly fixed on the future, in the spirit of unity, let's declare all together, with one voice, right here and right now, that Barack Obama is our candidate and he will be our president."

Pennsylvania was among the states short-circuited from a chance to announce its votes. But its delegates added their voices to the deafening chorus of ayes to Mrs. Clinton's motion to nominate Mr. Obama by acclamation.

After ruling that the motion had passed, House Speaker Nancy Pelosi declared him the party's nominee, climaxing an improbable political ascent launched on the national stage with a riveting speech at the last national party convention four years ago.

Mr. Biden's appearance at the podium was triumphant sequel to two also-ran bids for the top spot on the Democratic ticket, in 1988 and again this year. His address to the largest television audience of his long political career mixed populist autobiography with partisan jabs at the Republican with whom he said he shared "a friendship that goes beyond politics."

Turning his sights on Mr. McCain, he said, "As we gather here tonight, our country is less secure and more isolated than at any time in recent history. The Bush-McCain foreign policy has dug us into a very deep hole, with very few friends to help us climb out."

He portrayed his friend Mr. McCain as a handmaiden of oil companies and the Bush administration, insensitive to the needs of average workers.

"Should we trust John McCain's judgment when he says there can be no timelines to draw down our troops from Iraq -- that we must stay indefinitely?" he demanded, "Or should we listen to Barack Obama, who says shift responsibility to the Iraqis -- and set a time to bring our combat troops home?"

Moving to a series of other issues dividing the parties, claiming, with the vocal assistance of the crowd, that on each of them, "John McCain was wrong and Barack Obama was right."

Mr. Biden's speech was well-received by the partisan crowd, but the warm visual display that ended it was perhaps the more important message for the television audience. The four generations of Bidens who would join him and the head of their ticket were offered as a visual symbol of the family values his party would claim for its own.

During his speech, he introduced his mother, "Catherine Eugenia Finnegan Biden," and related that "when I got knocked down by guys bigger than me, she sent me back out and demanded that I bloody their noses so I could walk down the street the next day."

That pugnacious lesson emerged in much of a speech that reflected the traditional role of vice presidential candidate as attacker. In addition to his salvoes at Mr. McCain and President Bush, he also targeted Vice President Dick Cheney.

"For every American who is trying to do the right thing, for all those people in government who are honoring their pledge to uphold the law and respect our Constitution, no longer will the eight most dreaded words in the English language be: The vice president's office is on the phone."