DENVER -- Barack Obama stepped triumphantly into history Wednesday night, the first black American to win a major party presidential nomination, as thousands of Democrats transformed their convention hall into a joyful, shouting celebration.

Former rival Hillary Rodham Clinton asked delegates to the party convention to make their verdict 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" surged up from the convention floor as the outcome of a carefully scripted roll call of the states was announced.

Obama was across town in his hotel suite as the party handed him its top prize -- a ticket into the general election campaign against Republican Sen. John McCain. He was expected to briefly visit the Pepsi Center later in the evening to thank the delegates.

The polls showed a close race ahead, and Obama was hoping the party would leave its convention united despite the hard feelings remaining from a bruising primary campaign.

Former President Bill Clinton did his part, delivered a strong pitch for the man who outmaneuvered his wife for the nomination. "Everything I've learned in eight years as president and the work I've done since, in America and across the globe, has convinced me that Barack Obama is the man for this job," he said, to loud cheers.

Michelle Obama, watching from her seat in the balcony, stood and applauded as the former president praised her man.

The convention ends Thursday with Obama's acceptance speech, an event expected to draw a crowd of 75,000 at a nearby football stadium where an elaborate backdrop was under construction.

Obama's nomination was the main event of an evening that also included the installation of his choice of Delaware Sen. Joseph Biden as vice presidential running mate. Biden had the marquee time spot for his acceptance speech late Wednesday.

Hillary 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 after their long and often-bitter fight for the nomination.

Inside the convention hall, the outcome of the 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."

And Clinton did get hundreds of votes in the roll call -- 341 to Obama's 1,549 -- before she called for him to be approved by acclamation.

Polls show the campaign now is a close one between Obama and McCain, and both campaigns have been advertising in nearly a dozen battleground states for weeks.

The same surveys show a strong desire for change after eight years of the Bush administration, and Obama has 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.

"A lot of white workers ... and quite frankly a lot of union members believe he's the wrong race," AFL-CIO Secretary-Treasurer Richard Trumka told a breakfast meeting of Michigan delegates.

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.