Jae C. Hong/AP Photo

Democratic presidential nominee Barack Obama, right, appears with vice presidential nominee Joe Biden, left, Wednesday at the Democratic National Convention in Denver. Democrats swept away more than 200 years of history Wednesday and nominated Obama for the White House, a milestone punctuated by former President Bill Clinton's declaration that the first African-American standard bearer of a major party "is ready to be president of the United States."

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.

The son of a black Kenyan father and a white American mother is now one victory from becoming president of a nation where, just decades ago, many blacks were denied the vote.

Chants of "Obama" and "Yes we can" surged up from the convention floor as the outcome was announced.

Later, when their nominee paid a late-night visit to the hall, Obama embraced running mate Joe Biden and implored the delegates to help him "take back America" in the fall campaign against Republican Sen. John McCain of Arizona.

"Change in America doesn't start from the top down," he told the adoring crowd, "it starts from the bottom up."

Racial questioning

But even as he won the nomination, there was open talk in the convention city that Obama's 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 will face McCain, who will accept the Republican nomination next week in St. Paul, Minn.

Earlier, former rival Hillary Clinton asked the convention to interrupt its roll call of the states and make its verdict unanimous "in the spirit of unity, with the goal of victory." And they did, with a roar.

The polls show a close race ahead with McCain, a former Vietnam prisoner of war a few days shy of his 72nd birthday, and Obama was hoping Democrats would leave their convention united despite the hard feelings remaining from a bruising primary campaign that stretched over 18 months.

'Obama is the man'

Former President Bill Clinton did his part, delivering a strong pitch for the man who defeated 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.

And Obama, delighting the crowd with his appearance on stage, praised both Clintons and his wife for their prime-time speeches this week.

"If I'm not mistaken, Hillary Clinton rocked the house last night!" he shouted.

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.

Biden's speech

Biden, who has twice sought the presidency in his own right, won his place on Obama's ticket by acclamation.

In his acceptance speech, Biden said Obama was right about Iraq, a war he opposed from the start, and McCain was wrong.

"These times require more than a good soldier. They require a wise leader," Biden said. "A leader who can deliver change."

Obama isn't the first black man to seek the White House, but is the first with a chance to win it. Others, including Jesse Jackson in 1984 and 1988, tailored their appeals largely to blacks or lower-income voters of all races.

Obama's reach for political power and history was different, aimed at the broad American political middle. And his nomination, delivered so jubilantly, represents a gamble of sorts by the Democratic Party that a country founded by slave-owners and desegregated only in recent decades - and even then sometimes violently - is ready to place a black man in the Oval Office.

Jibes at McCain

Sen. John Kerry, the party's 2004 nominee, said Obama's victory shouldn't be a close call. In some of the strongest anti-McCain rhetoric of the week, he said his longtime friend is masquerading as a maverick. "The candidate who once promised a 'contest of ideas' now has nothing left but personal attacks," he said. "How insulting ... how pathetic ... how desperate."

Hillary Clinton's call for Obama to be approved by acclamation - midway through the traditional roll call of the states - was the culmination of an agreement worked out between the two camps to present a unified front after their 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 released her delegates amid shouts of "no," by disappointed supporters. 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.