Businessman Donald Trump was poised for an upset early Wednesday to become the 45th US president after engaging his Democratic rival Hillary Clinton in a fierce and what many described as an ugly campaign.

Trump had won 244 Electoral College votes compared with Clinton's 215 by 1:30 am Wednesday morning while Clinton had a slight lead in the popular vote.

Pennsylvania, with 20 electoral votes, and Michigan, with 16, were still too close to call, but Trump was leading in both states. A candidate needs 270 Electoral College votes to become a president.

Trump won Alabama, Arkansas, Florida, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Montana, Nebraska, North Carolina, North Dakota, Ohio, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, Utah, West Virginia and Wyoming, according to the Associated Press and Fox News.

Clinton won California, Colorado, Connecticut, Delaware, District of Columbia, Hawaii, Illinois, Maryland, Massachusetts, Nevada, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont, Virginia and Washington.

If elected, Trump would be inaugurated on Jan 20 to succeed President Barack Obama, a Democrat and the nation's first African-American president.

The win would come as a big surprise as many had expected an easy win for Clinton. But the mood in the Clinton headquarters was described as somber when Trump took the key states of Florida and Ohio.

Trump officially declared his candidacy for Republican presidential nomination on June 16, 2015 with a slogan to Make America Great Again. He accepted the nomination on July 21 this year by defeating 16 Republican rivals.

The real estate tycoon has run a campaign against the Washington establishment and political correctness and tapped into people's anger with the way the country has been going, on issues such as wage stagnation, health care and immigration.

About 46 million out of more than 200 million registered voters had cast their ballots in early voting, which was offered in 34 out of the 50 states and the District of Columbia.

Earlier this week, AFP reported that a Chinese monkey described as the "king of prophets" tipped Trump for the US presidency. The 5-year-old monkey in Hunan province reportedly also correctly predicted Portugal would win the 2016 European football championship in July.

Trump, who turned 70 on June 14, cast his vote at a school near his Manhattan residence on Tuesday morning, while Clinton started her day by casting her ballot in Chappaqua, New York, where she and her husband, former president Bill Clinton, have lived since 2001.

In the Tuesday election, Republicans also kept their majority in the House of Representatives.

The 2016 campaign has been mired in several major scandals. On July 23, WikiLeaks leaked 20,000 emails from the Democratic National Committee, showing a systemic bias against Clinton's major rival Bernie Sanders by the Democratic Party leadership. The revelation led to the resignation of DNC chairwoman Debbie Wasserman Schultz.

WikiLeaks later released more emails hacked from the Clinton campaign, revelations that triggered great concern among American public.

On Oct 7, tapes were leaked from Access Hollywood showing Trump bragging about sexual advances in 2005, causing a heavy blow to Trump in national polls compared to Clinton.

On Oct 28, FBI Director James Comey announced that the FBI would be restarting the investigation into Clinton's emails, slowing Clinton's momentum. But Comey told Congress on Nov 6 that there was no evidence in the emails that Clinton should face charges over her handling of classified information.

A New York Times/CBS News poll last Friday showed that 78 percent of Americans were disgusted with the 2016 campaign.

A Gallup poll released on Tuesday showed that Trump and Clinton headed into Election Day with the worst favorability ratings in history. The poll found that Trump's 61 percent unfavorable score was the worst in presidential polling since 1956, while Clinton's 52 percent unfavorable score was second-worst.

Allan Rivlin, CEO of Zen Political Research Center, dismissed the notion that a President-elect Trump would be authoritarian saying he would face resistance from his own party and complete resistance from the Democrats.

"Complete resistance from one party right now is enough to block something," he told a briefing at the Washington Foreign Press Center on Tuesday.

He also pointed out that the courts in the US are strong and designed to stop exactly such a move.

Unlike in previous presidential elections, there has been a lack of serious discussion and debate of US-China relations on the campaign trail.

Trump has promised to impose punitive tariffs on Chinese imports, a claim regarded as unrealistic given its serious violations of World Trade Organization rules, the opposition by US Congress and business community.

A report by the Council on Foreign Relations said that Trump wants to increase US military presence in and around the South China Sea; investigate and punish China for unfair trade practices; designate China a currency manipulator; and ratchet up the US deterrent against Chinese cyberattacks.

While Trump's foreign policy is largely unclear, he has expressed his willingness to meet North Korea leader Kim Jong-un, improve relations with Russia and be less interventionist, a contrast to his rival Clinton, whom many Chinese regard as hawkish and interventionist.

Jon Taylor, professor and chair of the department of political science at the University of St Thomas in Houston, said Trump has been upfront regarding his negative economic and military views toward China.

"I think that we will see him pushing the envelope of comfort regarding Sino-US relations," Taylor said. "His inexperience will likely mean that he will be very aggressive on trade issues and will challenge (China) in ways that will both reassure US allies and anger China regarding the US presence in the Pacific."

Douglas Spelman, senior adviser of the Kissinger Institute on China and the United States at the Wilson Center, said Trump's lack of foreign policy experience, in general and with China specifically, will hamstring his approach to US-China relations.

"He will likely aggressively criticize, in rhetoric and action, China's trade policies and its 'stealing' US jobs," Spelman said, citing Trump's earlier rhetoric of imposing 45 percent punitive tariffs on Chinese imports.

Zhiqun Zhu, professor of political science and international relations at Bucknell University in Pennsylvania, said President Obama has been lukewarm toward President Xi Jinping's proposition of a new model of great power relations.

"The new president should be more straightforward. If you like it, embrace it and work with China to build such a constructive relationship," he said.

[email protected]