The 47-year-old Democratic senator from Illinois sealed his victory by defeating Republican Sen. John McCain in a string of wins in hard-fought battleground states - Ohio, Florida, Virginia and Iowa.
A huge crowd thronged Grant Park in Chicago to cheer Obama's improbable triumph and await his first public speech as president-elect.
Obama and his running mate, Sen. Joseph Biden of Delaware, will take their oaths of office as president and vice president on Jan. 20, 2009.
As the 44th president, Obama will move into the Oval Office as leader of a country that is almost certainly in recession, and fighting two long wars, one in Iraq, the other in Afghanistan.
Earlier, the Illinois senator beat John McCain in Ohio on his way to building an insurmountable Electoral College advantage as he bid to become the first black president.
Fellow Democrats are gaining strength in both houses of Congress.
Obama's Ohio victory denied McCain particularly precious territory. No Republican has ever won the presidency without the state.
Earlier, Democrats picked up a Virginia Senate seat and elected a Missouri governor.
Obama swept to victories in traditionally Democratic states in the East and Midwest, while McCain countered in the safest of Republican territory.
That left the battlegrounds to settle the race: Florida, Ohio, Virginia, Pennsylvania and more. Most of them were customarily Republican, but Obama spent millions hoping to peel away enough to make him the 44th president.
Interviews with voters suggested that almost six in 10 women were backing Obama, and men leaned his way by a narrow margin. Just over half of whites supported McCain, giving him a slim advantage in a group that President Bush carried overwhelmingly in 2004.
The results of The Associated Press survey were based on a preliminary partial sample of nearly 10,000 voters in Election Day polls and in telephone interviews over the past week for early voters.
The same survey showed the economy was by far the top Election Day issue. Six in 10 voters said so, and none of the other top issues - energy, Iraq, terrorism and health care - was picked by more than one in 10.
The AP made its calls of individual states based on surveys of voters as they left the polls.
Obama had Vermont, Connecticut, Delaware, Illinois, Maine, Massachusetts, Maryland and New Jersey, as well as the District of Columbia, for 78 electoral votes. McCain had challenged in none of them.
McCain had Kentucky, Tennessee, Oklahoma and South Carolina, for 34 electoral votes. Obama conceded them from the outset.
The nationwide popular vote also favored Obama, who was gaining 53 percent to his rival's 47 percent.
The Senate seat that switched from Republican to Democrat was in Virginia, where former Gov. Mark Warner won his race to replace retiring Republican John Warner. The two men are not related.
Missouri's Attorney General, Jay Nixon was elected his state's governor, replacing a Republican, Matt Blunt, who retired rather than run again.
The White House was the main prize of the night on which 35 Senate seats and all 435 House seats were at stake. In both houses, Democrats hoped to pad their existing majorities, and Republicans braced for losses.
A dozen states elected governors, and ballots across the country were dotted with issues ranging from taxes to gay rights.
By tradition, the first handful of ballots were cast just after midnight in tiny Dixville Notch, N.H. Obama got 15 votes and McCain six.
They were the first of tens of millions in the race to gain 270 electoral votes and succeed George W. Bush on Jan. 20 as the 44th president.
An estimated 187 million voters were registered, and in an indication of interest in the battle for the White House, 40 million or so had already voted as Election Day dawned. Turnout was heavy. In Virginia, for example, officials estimated nearly 75 percent of eligible voters would cast ballots.
Obama awaited the results at home in Chicago after a marathon campaign across 21 months and 49 states. At 47, with only four years in the Senate, he sought election as one of the youngest presidents, and one of the least experienced in national political affairs.
That wasn't what set the Illinois senator apart, though - neither from his rivals nor from the 43 men who had served as president since the nation's founding more than two centuries ago. A black man, he confronted a previously unbreakable barrier as he campaigned on twin themes of change and hope in uncertain times.
McCain, a prisoner of war during Vietnam, a generation older than his rival at 72, waited in Arizona to learn the outcome of the election. It was his second try for the White House, following his defeat in the battle for the GOP nomination in 2000.
A conservative, he stressed his maverick's streak. And a Republican, he did what he could to separate himself from an unpopular president.
For the most part, the two presidential candidates and their running mates, Republican Gov. Sarah Palin of Alaska and Democratic Sen. Joe Biden of Delaware, spent weeks campaigning in states that went for Bush four years ago. Virginia, Florida, North Carolina, Ohio, Iowa, Colorado, New Mexico and Nevada drew most of their time. Pennsylvania also drew attention as McCain sought to invade traditionally Democratic turf.
McCain and Obama each won contested nominations - the Democrat outdistancing former first lady Hillary Rodham Clinton - and promptly set out to claim the mantle of change.
"I am not George W. Bush," McCain said in one debate.
Obama retorted that he might as well be, telling audiences in state after state that the Republican had voted with the president 90 percent of the time across eight years of the Bush administration.
After voting with her husband, the former president, Clinton called Bush "the lamest of lame ducks" and predicted that Obama would win and begin making presidential appointments and announcing economic policies within weeks.
The war in Iraq dominated the campaign early on, but by Election Day it had faded as an issue.
The economy mattered above all else, with millions facing home foreclosures, joblessness rising and Americans tallying the losses in their retirement accounts after a stock market plunge.
The race was easily the costliest in history, in excess of $1 billion, more after the congressional campaigns were counted.
McCain accepted federal matching funds, and was limited to $84 million for the fall campaign.
After first saying he would go along, Obama reversed course, then raised and spent multiples of what his rival was allowed.
McCain sought to make an issue of that, saying Obama had broken his word to the public. For weeks on end, he could not match his rival's television advertising onslaught.
Figures through mid-October showed Obama had spent roughly $240 million on television and radio advertisements.
McCain had shelled out about $115 million, and the Republican National Committee an additional $80 million on his behalf.
In the battle for Congress, Democrats began the night with a 51-49 majority in the Senate, including two independents. Their majority in the House was 235-199, with one vacancy.
In both cases, Republicans fought to overcome a financial disadvantage as well as numerous retirements.
The governor's races included open seats in North Carolina, Delaware and Missouri.
The ballot issues ran from a measure to ban abortion in South Dakota to proposals outlawing affirmative action in Colorado and Nebraska. Three states voted on gay marriage.
