DENVER (AP) -- 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.

Competing 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 American doesn't start from the top down," he told the adoring crowd, "it starts from the bottom up."

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.

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

Obama, delighting the crowd with his appearance on stage, praised both Clintons as well as his wife, Michelle, for their prime time speeches this week.

The convention ends today with Obama's acceptance speech, an event expected to draw a crowd of 75,000 at a nearby football stadium.