President George W. Bush is on the verge of winning a second term in the White House on last night's election count.
But Democrats said uncounted provisional votes in Ohio could change the outcome.
Mr Bush was on track to win a rare majority, 51 per cent, of the popular vote - a decisive endorsement of his policies in Iraq, the hottest issue of the election campaign.
The President won Florida, the state he secured four years ago only after a Supreme Court decision.
His challenger, Senator John Kerry, hung on to the Democrat prize of Pennsylvania, but needed also to win Ohio to swing the balance his way.
On election night results, Mr Bush won in Ohio by 145,000 votes.
But Democrats are holding out for the count of 250,000 special votes, not due to be tallied for 11 days.
Democrat vice-presidential nominee John Edwards said the party would "fight for every vote".
Senator Edwards told supporters in Boston that he and Mr Kerry would not give in.
"John Kerry and I have made a promise to the American people that with this election every vote would count and every vote would be counted," said Mr Edwards, from North Carolina.
"Tonight we are keeping our word, and we will fight for every vote."
Two television networks projected that Mr Bush would win Ohio, but three others did not make a prediction.
Late last night White House chief of staff Andy Card declared that Mr Bush had won a "convincing electoral college victory."
"In Ohio, President Bush has a lead of at least 140,000 votes, " Mr Card said.
"The Secretary of State's office has informed us that this margin is statistically insurmountable, even after the provisional ballots are considered."
Heavy voter turnout was reported nationwide, and few major voting glitches were recorded in the final act of a presidential campaign marked by deep divisions between the contenders and their supporters over the war in Iraq, the fight against terrorism and the economy.
Predictions of voter challenges and election chaos mostly did not come true in an election where turnout was expected to substantially exceed the 105 million who voted in 2000.
Without Ohio, Mr Kerry would need to win Nevada, won by Mr Bush in 2000, and hold the undecided states of Wisconsin, Iowa and New Mexico for a 269-269 electoral tie.
That would throw the race to the Republican-led House of Representatives, which would be almost certain to opt for Mr Bush.
Late last night, Mr Bush held a lead of nearly 4 million votes over Mr Kerry, after 89 per cent of polling places had reported.
With 270 electoral votes needed to win the White House, Mr Bush had a provisional 274 electoral votes if Ohio's 20 were counted as his. Mr Kerry had 252 votes.
Kerry campaign manager Mary Beth Cahill said that the vote count in Ohio had not been completed. "There are more than 250,000 remaining votes to be counted," she said.
"We believe when they are, John Kerry will win Ohio."
An unknown number of those votes were provisional ballots issued to voters when their registration was challenged.
Ohio Secretary of State Ken Blackwell said those votes would not be counted for 11 days.
"If the number of votes that make up the difference between the two candidates is fewer than the number of provisional ballots, then I would say everybody should just take a deep breath and relax because we're not going to start counting those ballots until the eleventh day after the election," he said.
Voters also decided on governorships in 11 states, and left the Republicans in control of the Senate and House of Representatives.
Republicans gained Senate seats in North Carolina, Georgia, South Carolina and Louisiana, and Democrats picked up seats in Illinois and Colorado.
A few disputes flared in swing states as officials began counting votes. A Philadelphia judge blocked the counting of up to 12,000 absentee ballots in the city until he holds a hearing today after a complaint brought by the Republican Party.
Thousands of people were still queuing to vote more than two hours after the polls closed in Ohio, and officials said they would be allowed to stay and vote as long as they were in the queue at closing time.
Mr Bush won one-time battlegrounds such as West Virginia, Arizona and Missouri, and Mr Kerry took New Jersey.
Mr Bush and Mr Kerry cast votes in their respective home states of Texas and Massachusetts earlier in the day then settled in for a long night of watching and waiting.
Mr Bush, who watched the results in the White House with his family, including his father, former President George Bush, said he was confident of victory.
"We're very upbeat, thank you," Mr Bush told reporters. "I believe I will win."
Mr Kerry, watching the results in his hometown of Boston, did not make an appearance before reporters but sent out aides to predict a win.
Officials in Florida, where a bitterly disputed 2000 recount ultimately handed the White House to Mr Bush, reported long lines but no early voting problems.
The American dollar lost ground and US Treasury prices rose early yesterday after the uncertainty in Ohio made the market more cautious about betting on a Bush victory.
Earlier, US oil prices rebounded sharply from early lows, pushed up by speculation that Mr Bush would win re-election.
- REUTERS
Herald Feature: US Election
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Bush on verge of winning, Democrats wait for special votes
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