Former rival Hillary Rodham 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.
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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.
"The Clinton and Obama camps orchestrated a roll call that pleased most everyone, upset almost no one and provided an historic moment for the nightly newscasts," said CBSNews.com senior political editor Vaughn Ververs. "It was another attempt by the party to show a united front coming out of this convention, something that looks more possible by the day."
Obama, 47 and in his first Senate term, carries the Democrats' hopes of recapturing the White House into the fall campaign against Sen.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 tonight will also include the delegates' acceptance of Obama's choice of Delaware Sen.as vice presidential running mate. Biden has the marquee time spot for his acceptance speech late Wednesday.
Former President Clinton also has 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.
Anticipating Wednesday night's focus on national security at the convention, Republican John McCain contended in a new TV ad that Obama showed he was "dangerously unprepared" for the White House when he described Iran as a "tiny" nation that didn't pose a serious threat.
"Iran. Radical Islamic government. Known sponsors of terrorism. Developing nuclear capabilities to 'generate power' but threatening to eliminate Israel," says the ad, which was being run in key states. "Terrorism, destroying Israel - those aren't 'serious threats"'?
Missing from the ad was the context of Obama's remarks last May in which he compared Iran and other adversarial governments to the superpower Soviet Union. "They don't pose a serious threat to us the way the Soviet Union posed a threat to us," he said in arguing for talks with Iran. "You know, Iran, they spend one-100th of what we spend on the military. If Iran ever tried to pose a serious threat to us, they wouldn't stand a chance."
Other Republicans, meanwhile, also struggled for a bit of the spotlight. On Wednesday, former New York Mayor Rudy Giuliani, the keynote speaker for the Republican convention next week, said that Hillary Clinton never told delegates that Obama was prepared for the presidency in her speech to the convention last night.
"Nowhere in that speech did she answer the question about his character, his ability to lead, the things that are at issue here," Giuliani said on CBS News' The Early Show. "And until she does, you're going to have a lot of Hillary Clinton supporters that are either not going to vote ... or are going to vote for John McCain."
Former Massachusetts Gov. Mitt Romney, a potential running mate for McCain, also came to Denver and said Tuesday, "Barack Obama is a charming and fine person with a lovely family, but he's not ready to be president."