DENVER — Barack Obama, claiming a prize never held by a black American, swept to the Democratic presidential nomination on Wednesday as thousands of national convention delegates stood and cheered his improbable triumph.

Former rival Hillary Clinton asked the convention delegates to make it unanimous “in the spirit of unity, with the goal of victory.” And they did, with a roar.

Competing chants of “Obama” and “Yes we can” floated up from the convention floor as Obama’s victory was sealed.

Obama was across town as the delegates he won in the primaries of winter and spring cast their votes. Aides left open the possibility that he would briefly visit the Pepsi Center to thank his supporters, a routine event at recent national conventions. His formal acceptance speech Thursday night was expected to draw a crowd of 75,000 at a nearby football stadium where an elaborate backdrop was under construction.

Clinton’s call for Obama to be approved by acclamation — midway through the traditional roll call of the states — was the culmination of a painstaking agreement worked out between the two camps to present a unified front.

Obama, 47 and in his first Senate term, carries the Democrats’ hopes of recapturing the White House into the fall campaign against Sen. John McCain and the Republicans.

Inside the convention hall, the outcome of the traditional roll call of the states was never in doubt, only its mechanics.

“No matter where we stood at the beginning of this campaign, Democrats stand together today,” declared Rep. Debbie Wasserman Schultz of Florida, a former Clinton supporter who delivered a nominating speech for Obama.

“We believe passionately in Barack Obama’s message of changing the direction of our country,” she said.

Earlier in the day, Clinton formally released her delegates amid shouts of “no,” by disappointed supporters. “She doesn’t have the right to release us,” said Massachusetts delegate Nancy Saboori. “We’re not little kids to be told what to do in a half-hour.”

Polls showed the campaign was a close one between Obama and McCain.

The same surveys showed a strong desire for change after eight years of the Bush administration, and Obama pledged an end to the war in Iraq and a fresh economic policy.

But even as he awaited his nomination, there was open talk in the convention city that his race remained a stumbling block to winning the White House.

The convention program also included the delegates’ acceptance of Obama’s choice of Delaware Sen. Joseph Biden as vice presidential running mate. Biden had the marquee time spot for his acceptance speech late Wednesday.

Former President Clinton also had a turn at the podium, this time in a supporting role for the man who defeated his wife in a bruising battle for the nomination.

Obama’s nomination sealed a political ascent as astonishing as any other in recent memory — made all the more so by his race, in a nation founded by slave owners.

The son of a white mother from Kansas and a black father from Kenya whom he barely knew, he attended college and Harvard Law School. In between was a turn as a $12,000-a-year community worker on the streets of Chicago.

He won his seat in the Illinois Legislature in 1996. But his first bid for higher office, a brash challenge to Rep. Bobby Rush in an inner-city Chicago congressional district, ended in failure in 2000.

Four years later, as a candidate for the Senate, he dazzled with a keynote address at the 2004 Democratic National Convention, then won his election. He announced his presidential candidacy a scant two years after arriving in Washington.

With his gifts as a speaker, his astounding ability to raise funds on the Internet and an unmatched ground operation pieced together by political veterans, he won the first test, the Iowa caucuses, on Jan. 3

Clinton rebounded to win the New Hampshire primary five days later, and the two were soon matched in a grueling battle for the nomination that was not settled until the primaries ended in June.

“The journey will be difficult. The road will be long,” he said then as he pivoted to confront McCain.