Democratic Senator Barack Obama is projected to have won two key battleground states, putting him within sight of the presidency.
His projected win in Ohio, won by the Republicans in 2004, is a crucial stepping stone to the White House.
He also took Pennsylvania - one of John McCain's main targets.
Mr Obama now needs only to win California and a couple of other solid Democratic states. Analysts said anything else would be a major shock.
However, the popular vote remains close. At 0345 GMT it stood at 50.7% for the Democratic senator from Illinois, against 48.2% for Arizona senator Mr McCain.
And a number of swing states looked like going to the wire. In Indiana and North Carolina, with most of the vote counted, there was less than 0.5% between the two candidates.
The main developments include:
An Obama win in Ohio, if confirmed, would be an important gain. The state, with its 20 electoral college votes, is a key battleground. It was a vital win for George W Bush in 2000 and 2004.
LOSSES AND GAINS
New Mexico and Iowa were also Republican last time round, though much less influential, with five and seven electoral college votes respectively.
All of the other results so far have gone according to expectations, and followed the same pattern as in 2004.
But Pennsylvania was significant, because Mr McCain was hoping to score an upset there, and had spent a lot of time campaigning in the state.
BBC North America editor Justin Webb said this one result could mean the night "has ended relatively early".
Mr McCain had also hoped to pick up New Hampshire.
Momentous day
Several key states are reporting a heavy turnout. A total of 130 million Americans are expected to vote.
If that figure is confirmed, turnout will be higher than for any election since 1960.
There were reports of glitches with electronic voting machines in some states, including Florida, Ohio and Virginia - though not on a big scale.
Mr Obama and his wife voted at a polling station in Chicago, Illinois, with their two daughters.
"When polls close, the journey ends but voting with my daughters, that was a big deal," he said afterwards.
Mr McCain cast his ballot in Phoenix, Arizona.
Unusually, both candidates continued to campaign during polling day - Mr Obama in Indiana and his rival in Colorado and New Mexico.
Mr McCain, 72, urged supporters to "get out there and vote", and "drag" their neighbours to polling stations if need be.
Many Americans said they felt they were voting in a historic election, not least because of the possibility of choosing the first African-American president.
Faton Fall, 40, a black voter queuing at a Baptist church in Chicago, said: "It means a lot to me. I'm overwhelmed. I can't say more."
There are also elections to renew the entire US House of Representatives and a third of US Senate seats.
Democrats are expected to expand majorities in both chambers.
They need to gain nine Senate seats to reach a 60-seat majority that would give them extra legislative power.
Daunting tasks
The final Reuters/C-SPAN/Zogby poll of the election published on Tuesday found likely voters favoured Mr Obama by 11 points over Mr McCain, 54-43%.
Whoever becomes the 44th president of the US will inherit a daunting series of challenges, including wars in Iraq and Afghanistan, and a global financial crisis.
Under the US Electoral College system, states are allocated votes based on their representation in Congress.
In almost every state, the winner gets all these college votes.
To become president, a candidate needs to win a majority across the country - 270 college votes out of a possible 538.
The presidential election has been the most expensive in US history - costing $2.4bn, according to the non-partisan Center for Responsive Politics.
^ Back to top | BBC Sport Home | BBC Homepage | Contact us | Help | ©
~RS~q~RS~~RS~z~RS~43~RS~)