Donald Trump has been elected president, capping a remarkable comeback four years after he was voted out of the White House and ushering in a new American leadership likely to test democratic institutions at home and relations abroad.

Trump, 78, recaptured the White House on Wednesday by securing more than the 270 Electoral College votes needed to win the presidency, Edison Research projected, following a campaign of dark rhetoric that deepened the polarisation in the country.

The former president's victory in the swing state of Wisconsin pushed him over the threshold. 

Trump had won 279 electoral votes to Harris' 223 with several states yet to be counted as of 5.45am US eastern time (1045 GMT).

He also led Harris by about five million votes in the popular count.

Donald Trump, pictured with wife Melania Trump, was flanked by his family as he claimed victory. (AP PHOTO)

"America has given us an unprecedented and powerful mandate," Trump said early on Wednesday to a roaring crowd of supporters at the Palm Beach County Convention Center in Florida, his family by his side.

Trump's political career had appeared to be over after his false claims of election fraud led a mob of supporters to storm the US Capitol on January 6, 2021, in a failed bid to overturn his 2020 defeat.

But he swept away challengers inside his Republican Party and then beat Democratic candidate Kamala Harris by capitalising on voter concerns about high prices and what Trump claimed, without evidence, was a rise in crime due to illegal immigration.

Harris did not speak to supporters who had gathered at her alma mater Howard University. 

Her campaign co-chair, Cedric Richmond, briefly addressed the crowd after midnight, saying Harris would speak publicly later on Wednesday.

"We still have votes to count," he said.

Republicans won a US Senate majority, but neither party appeared to have an edge in the fight for control of the House of Representatives where Republicans currently hold a narrow majority.

Voters identified jobs and the economy as the country's most pressing problem, according to Reuters/Ipsos opinion polls. 

Many Americans remained frustrated by higher prices even amid record-high stock markets, fast-growing wages and low unemployment. 

Voters identified jobs and the economy as the country's most pressing problem, polls show. (EPA PHOTO)

With the administration of President Joe Biden taking much of the blame, a majority of voters said they trusted Trump more than Harris to address the issue.

Hispanics, traditionally Democratic voters, and lower-income households hit hardest by inflation helped fuel Trump's election victory. 

His loyal base of rural, white and non-college-educated voters again showed up in force.

Trump prevailed despite persistently low approval ratings. 

Impeached twice, he has been criminally indicted four times and found civilly liable for sexual abuse and defamation. 

In May, Trump was convicted by a New York jury of falsifying business records to cover up hush money payments to a porn star.

His victory will have major implications for US trade and climate change policies, the war in Ukraine, Americans' taxes and immigration.

His tariff proposals could spark a fiercer trade war with China and US allies, while his pledges to reduce corporate taxes and implement a spate of new cuts could balloon US debt, economists say.

Trump has promised to launch a mass deportation campaign targeting immigrants in the country illegally.

He has said he wants the authority to fire civil servants he views as disloyal. 

His opponents fear he will turn the Justice Department and other federal law enforcement agencies into political weapons to investigate perceived enemies.

Vice President Kamala Harris had warned that Donald Trump posed a danger to democracy. (AP PHOTO)

A second Trump presidency could drive a bigger wedge between Democrats and Republicans on issues such as race, gender, what and how children are taught, and reproductive rights.

Vice President Harris fell short in her 15-week sprint as a candidate, failing to galvanise enough support to defeat Trump, who occupied the White House from 2017-2021, or to allay voters' concerns about the economy and immigration.

Harris had warned that Trump wanted unchecked presidential power and posed a danger to democracy.

Despite legal woes and controversies, Trump is only the second former president to win a second term after leaving the White House. 

In May, Trump was convicted by a New York jury of falsifying business records to cover up hush money payments to a porn star. 

Two months later, a would-be assassin's bullet grazed his right ear during a campaign rally, exacerbating fears about political violence. 

Another assassination attempt was thwarted in September at his Florida golf course. 

Barely eight days after the July shooting, Biden, 81, dropped out of the race, finally bowing to weeks of pressure from his fellow Democrats after a poor performance during his debate with Trump called into question his mental acuity and the viability of his re-election bid.

Harris re-energised despondent Democrats and raised more than $US1 billion ($A1.5 billion) in less than three months while erasing what had been a solid Trump lead in opinion polls.

Her financial advantage was partly countered by the intervention of the world's richest man, Elon Musk, who poured more than $US100 million ($A152 million) into a super PAC mobilising Trump voters and used his social media site X to amplify pro-Trump messaging.

By Steve Holland, Nandita Bose, Stephanie Kelly and Joseph Ax in West Palm Beach