Biden, the Democratic candidate and a career politician, has had a strong and consistent lead in national polls.

But the Republican Trump is close in enough swing states to possibly piece together the 270 state-by-state Electoral College votes needed to hold on to the presidency, which he won in a surprise 2016 election result despite losing the popular vote to Democrat Hillary Clinton by nearly 3 million ballots.

However, it may be days before the result is known, especially if legal challenges focused on postal ballots are accepted in the event of a tight race.

There is also fear about possible unrest. Several cities are boarding up buildings in anticipation of possible protests, including around the White House and in New York City.

Polls opened in some Eastern states at 6 a.m. EST (1100 GMT). The most closely watched results will start to trickle in after 7 p.m. EST (2400 GMT) when polls close in states such as Georgia, though definitive national results could take days if the contest is tight.

The voting caps a campaign dominated by the coronavirus pandemic that has killed more than 231,000 people in the United States and put millions out of work. The country has also been shaken by nationwide protests over racial injustice against Black Americans.

Biden, who has framed the contest as a referendum on Trump's handling of the virus, promised a renewed effort to combat the health crisis, fix the economy and bridge America's political divide.

Although more than 99 million Americans have already voted in early, in-person voting or by mail, Election Day marks a deadline for votes to be cast and still serves as a ritual for those who go to the polls on the first Tuesday after the first Monday in November.

The early vote total, as reported by the US Elections Project at the University of Florida, set a record and underscored the intense interest in the election and concerns about voting in person on Election Day during a pandemic.

The number was equal to 72.3 percent of the entire turnout in 2016 and experts predicted turnout would easily surpass the 138 million who voted in 2016.

The election has already prompted a wave of litigation over whether to adjust voting rules in light of the COVID-19 pandemic. On Monday, a federal judge in Texas rejected a Republican bid to throw out about 127,000 votes already cast at drive-through voting sites in the Democratic-leaning Houston area.

Trump has questioned the integrity of the election results for months, making unsubstantiated claims that mail-in voting is rife with fraud and refusing to commit to a peaceful transfer of power if he loses.

Trump has also said votes should only be counted through election night, even though many states often take days or weeks to tally ballots.

Voters on Tuesday will also decide which political party will control the US Congress for the next two years, with Democrats pushing to recapture a Senate majority and expected to retain their control of the House of Representatives.

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