Kamala Harris appears to be on track to lose her bid to become America's first female president, with her Republican rival Donald Trump expected to win all key battleground states.

The vice president has not yet conceded, although forecasts from the BBC's US partner CBS suggest Trump is on the precipice of a historic victory.

Harris canceled her expected election night appearance at Howard University in Washington DC, where she was studying, after Trump gained momentum as results began to come in.

The Republican then swept the key battleground states of Pennsylvania, North Carolina and Georgia, the projects of the US BBC partner CBS.

Trump is also expected to win in other important swing states, including Wisconsin and Michigan.

As expected, Trump won in conservative strongholds from Florida to Idaho, while Harris won liberal states from New York to California, CBS predicts.

The Democratic candidate was scheduled to address her supporters, but campaign co-chair Cedric Richmond announced shortly after midnight that she would not be attending.

The party atmosphere from a few hours earlier at Howard had already deteriorated when two swing states were declared for Trump.

From Harris' headquarters, Democratic fundraiser Lindy Li told the BBC that things were “pretty bleak at the moment.”

Vice President Harris, 60, only became the Democratic Party's nominee in July after President Joe Biden withdrew from the race due to pressure within the party. Had she claimed victory, the former California senator would have been the first woman, Black woman and South Asian American to win the presidency.

But CBS exit poll data suggests that the Democratic candidate, who has campaigned heavily on abortion rights, may have underperformed with women.

According to the figures, around 54% of female voters cast their votes for her. But Joe Biden won the support of 57% of women in 2020.

Black and Latino voters also appeared slightly less likely to support Harris than Biden four years ago, according to Associated Press polling data.

About 86 million voters cast their ballots early during one of the most tumultuous election campaigns in modern American history.

The Republican Party appears to have experienced a resurgence across the country, winning a number of key congressional races in key states. On Tuesday evening, control of the US House of Representatives and the Senate was up for debate.

CBS predicts Republicans will win control of the Senate after wresting two seats from Democrats in West Virginia and Ohio and mounting a major challenge in Texas.

Neither party appeared to have an overall advantage in the House of Representatives, which is narrowly controlled by Republicans.

If the party does regain control of both chambers, Trump would have an easier time pushing through his agenda, which includes mass deportations of illegal immigrants and sweeping tax cuts.

Both sides had armies of lawyers ready to mount legal challenges on and after Election Day.

Law enforcement agencies across the country were also on high alert for possible violence.

According to CBS, around 30 fake bomb threats were targeted at polling places across the country on Tuesday, more than half of them in the state of Georgia alone.