Less than an hour after sealing his hold on the White House, Obama and his wife, Michelle, stepped onto the stage holding the hands of their two daughters at a massive victory rally in Chicago's Grant Park.

"If there is anyone out there who still doubts that America is a place where all things are possible; who still wonders if the dream of our founders is alive in our time; who still questions the power of our democracy, tonight is your answer," the new president said.

He spoke warmly of McCain, the 72-year-old Arizona senator who was his rival in the longest and most costly presidential campaign in American history.

After McCain called Obama to concede victory, he spoke graciously of Obama at an outdoor rally in Arizona, commending the president-elect on his victory and emphasizing that he understood its special importance to African-Americans.

"The American people have spoken, and spoken clearly," McCain told disappointed supporters in Arizona, many who booed and growled as he called for the nation to unify behind the victor and his running mate, Joe Biden.

Obama, a 47-year-old Illinois senator and son of a white mother from Kansas and an African father from Kenya, mined a deep vein of national discontent, promising Americans hope and change throughout a nearly flawless 21-month campaign for the White House.

Obama stepped through a door opened 145 years ago when Abraham Lincoln, a fellow Illinois politician, issued the Emancipation Proclamation that freed African-Americans from enslavement in the rebellious South in the midst of a wrenching civil war.

The powerful orator lays claim to the White House on Jan. 20, only 43 years after the country enacted a law that banned the disenfranchisement of blacks in many Southern states where poll taxes and literacy tests were common at the time.

Cautioning Americans that the nation's problems were manifest, Obama said: "The road ahead will be long. Our climb will be steep. We may not get there in one year or even one term, but America I have never been more hopeful than I am tonight that we will get there. I promise you we as a people will get there."

Obama heaped praise on his political strategists who crafted a campaign that took him deep into Republican territory and produced victory in the most important toss-up states, including Florida and Ohio.

The unstoppable Obama political engine turned the Internet into a money machine as volunteers and paid staff nationwide expanded voter rolls to record numbers.

The Obama campaign was so flush that it was able to spend roughly $240 million on television and radio ads through the middle of last month. His extraordinary fundraising capabilities vanquished primary rival Sen. Hillary Rodham Clinton, who had been seen as the party's inevitable nominee.

After Obama's victory Clinton called her former rival to promise her full support and congratulated Americans for making him the 44th U.S. president.

"In quiet, solitary acts of citizenship, American voters gave voice to their hopes and their values, voted for change, and refused to be invisible any longer," she said in a statement.

An Obama presidency offers the prospect of a new style and tone in American foreign policy. He has said he will try to withdraw U.S. troops from Iraq in 16 months and has called for a new opening to U.S. adversaries, such as Iran and Cuba. He has urged the closing of the Guantanamo Bay prison and favors cap-and-trade systems to reduce global warming.

Internationally, Obama is hugely popular - a sharp contrast to Bush. Part of his appeal is his personal story that highlights American multiculturalism: besides his Kenyan father he has a half-sister who is the daughter of an Indonesian.

With victories in Florida, Ohio, Pennsylvania, Virginia and other battleground states, Obama built a commanding lead over McCain after surging in the polls in the midst of a national financial crisis. He and his fellow Democrats sought to link McCain to the unpopular George W. Bush.

Shortly after 2 a.m. (0700 GMT) in the eastern United States, The Associated Press count showed Obama with 349 electoral votes, well over the 270 needed for victory. McCain had 147 after winning states that comprised the normal Republican base, including Texas and most of the South as well as several in the Midwest and Rocky Mountain west. Electoral votes are apportioned to the states roughly equivalent to their populations.

By comparison, Bush won the White House twice, and never tallied more than 286 electoral votes.

Obama soared into the national spotlight with his electrifying speech at the 2004 Democratic National Convention, when he was making his first run for the Senate and polishing his message of unity in a country that was mired in partisan anger.

Democrats also were expanding their majorities in both chambers of Congress.

Cheering, screaming and waving flags, an estimated 50,000-plus Obama supporters welcomed his election in a delirious victory celebration in the senator's hometown.

They crammed into Grant Park to be a part of something that would be remembered for generations.

The downtown Chicago park, where police fought anti-war protesters during the turbulent 1968 Democratic convention, was transformed on an unseasonably balmy night by white tents and a stage lined with American flags and hung with red, white and blue bunting.

Lighted windows in the skyscrapers lining the park added to the festive atmosphere, spelling out "USA" and "Vote 2008."

Watching the results on a jumbo TV screen, the crowd erupted in cheers each time Obama put yet another state in his victory column.

The assembled backers were treated to one of Obama's most uplifting speeches.

"To those who would tear this world down we will defeat you. To those who seek peace and security, we support you," he said. "And to all those who have wondered if America's beacon still burns as bright tonight, we proved once more that the true strength of our nation comes not from our the might of our arms or the scale of our wealth, but from the enduring power of our ideals: democracy, liberty, opportunity and unyielding hope."

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