Republican nominee Donald Trump is projected to have won several key victories against Democratic rival Hillary Clinton as the race for US president pushes towards a close.

Projections show Trump winning the battleground states of Ohio, Florida, North Carolina and Pennsylvania - four crucial states in the race for the Oval Office. 

At 266 Trump is now only four Electoral College votes away from clinching the White House. It takes 270 votes to win the presidency, and Clinton only has 218. 

At Clinton headquarters in New York early on Wednesday morning, Clinton campaign manager John Podesta told supporters to go home and get some sleep.

"Several states are too close to call so we're not going to have anything more to say tonight," he said.

Clinton carried Virginia, Colorado and California. Her campaign had expected easy victories there, but the states took on new urgency as Trump picked up votes elsewhere.

The uncertainty sent Dow futures and Asian markets tumbling, reflecting investor concern over what a Trump presidency might mean for the economy and trade.

Projected election results
Clinton wins:
Maine, Nevada, Colorado, Washington, Oregon, Hawaii, California, Virginia, New Mexico, Illinois, New York, Rhode Island, Massachusetts, Connecticut, Maryland, New Jersey, Vermont, Delaware and the District of Columbia
Trump wins:
AlPennsylvania, Utah, Iowa, Georgia, Wyoming, North Carolina, Idaho, Florida, Ohio, Missouri, Montana, Louisiana, Texas, Nebraska, Kansas, South Dakota, North Dakota, Texas, Wyoming, Mississippi, Alabama, South Carolina, Tennessee, Oklahoma, West Virginia, Kentucky, and Indiana

As Clinton's team anxiously waited for results to roll in, the candidate tweeted to supporters, "Whatever happens tonight, thank you for everything."

Florida is the largest of the “swing states” with 29 electoral votes. 

The sunshine state is a “make or break” state for Trump, according to analyst Allan Lichtman, who has correctly predicted the outcome of the elections for the past 30 years.

Al Jazeera's Gabriel Elizondo, reporting from Columbus, Ohio, said that the candidate that wins Ohio traditionally wins the election. 

"If you win Ohio, you win the presidency."

US television networks have projected that Republicans will retain control of the lower chamber of Congress known as the House of Representatives, where all 435 seats were up for grabs.

While the state of Ohio was blue in the two previous elections, Tuesday's result was not an unexpected loss for Clinton.

Eighty percent of the population in the state of Ohio is white - compared to 65 percent nationally - a demographic that favoured Trump.

Supporters of Republican presidential candidate Donald Trump cheer as they watch election returns during an election night rally [The Associated Press]

Trump hinted again on Tuesday that he would refuse to accept election results if he loses.

"We're going to see how things play out," Trump told Fox News on Tuesday, alleging that there had been voting irregularities. "I want to see everything honest."

Trump filed a lawsuit on Tuesday against the Clark County Registrar of Voters in the state of Nevada, alleging that early voting stations were open longer than the designated time.

A Nevada judge denied Trump's request for state voting records on the grounds that the request would potentially put voters at risk of "ridicule and harassment".

Radically different visions

As a nervous world watched and waited, Americans chose between radically different visions of the future offered by Clinton and Trump.

The 69-year-old former first lady, senator and secretary of state - who is backed by incumbent President Barack Obama - on Monday urged the country to unite and vote for "a hopeful, inclusive, big-hearted America" in her last effort to woo voters.

Trump, meanwhile, pressed his message with voters who feel left behind by globalisation and social change, wrapping up with a flourish on his protectionist slogan of "America first".

Promising to end "years of betrayal", to tear up free trade deals, to seal the border, to halt the drug trade and to subject Syrian refugees to "extreme vetting", Trump told his supporters in New Hampshire: "I am with you and I will fight for you and we will win."

Trump has won nearly all of the main swing states, including Florida, Ohio and Pennsylvania [Reuters]

Trump has repeatedly warned that a "corrupt Washington and media elite" is seeking to rig the race and he said last month that he may not concede defeat if he thinks voting is unfair.

"When he was asked: will he concede the elections, his response was 'We will see'," Saloomey said.

The Al Jazeera correspondent said that Trump has continued with the same theme that has got him this far.

"He has showed his lack of faith in the polls that show him slightly behind Hillary Clinton. He has made some advances in areas that we call Rust Belt here in the US - all the industrial cities and towns where there are lots of working-class voters.

"This has been his strongest area of support," she said.

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Clinton has pushed a more optimistic vision, despite a wobble in the final weeks of her campaign when the FBI reopened an investigation into whether she had put US secrets at risk by using a private email server - only to close the probe again on Sunday.

The email investigation allowed Trump to recover ground lost in a series of recent scandals.

Since announcing his presidential campaign in June 2015, Trump, a billionaire businessman from New York, has consistently alienated minority groups, refused to release his tax returns, and remained seemingly unapologetic for leaked tapes in which he brags about sexually assaulting women.

Voters are also electing candidates for 34 seats in the 100-member Senate and the entire 435-member House of Representatives. Both the House and Senate are now controlled by the Republicans.

With a dominance in Congress, the Republicans frustrated outgoing President Obama by rejecting many of his key legislative agendas.

A Trump victory, along with a Republican Congress, could mean a swift end for Obama's Obamacare health reforms.

To win control of the Senate, Democrats would have to score a net gain of five seats. Republicans currently hold 54 Senate seats to 44 Democratic seats and two independents who align themselves with Democrats.

Source: Al Jazeera News