It's been a campaign for the history books, upended by the Covid-19 pandemic and taking place in a nation on edge after a summer of civil unrest. On the eve of voting, here are some of the highlights in review:
- Biden's primary comeback -
Joe Biden got off to a bad start in the battle to become the Democratic Party candidate.
In the first primary in Iowa in February, he came a distant fourth in a caucus hit by delayed results.
His campaign appeared on the edge of collapse as left-winger Bernie Sanders gathered support in the following primaries, before Biden scored a stunning recovery in South Carolina, thanks largely to African Americans voters.
Sanders pulled out of the race in April, and the Democratic rivals rapidly united behind Biden.
Donald Trump became the Republicans' nominee in March without serious opposition.
- Covid changes everything -
The spread of the Covid-19 pandemic from March upended the normal campaign schedule, putting a stop to all large rallies and meetings.
Biden, 77, has spent much of the year in his house in Wilmington, Delaware, as a patchwork of shutdowns across the country plunged the economy into crisis.
Trump, 74, started a daily briefing on the pandemic at the White House, often using it as a surrogate campaign event. He later halted the briefings as the death toll soared.
In mid-June, Trump tried to revive his ground campaign with a big rally in Tulsa, Oklahoma, but poor attendance made it a fiasco.
- The George Floyd protests -
The police killing of George Floyd, an unarmed African American, in the Midwestern city of Minneapolis in May touched off months of mass anti-racism protests across the United States -- some of them turning violent.
With much of the nation locked down, and the campaign muted by the pandemic, tens of thousands filled the streets to demand an end to police brutality and systemic racism -- a call largely embraced by the Democrat Biden.
Trump tacked in the opposite direction, seizing on the civil unrest as the work of "anarchists" and pushing a law-and-order message that would remain central to his reelection campaign.
- Conventions, reinvented -
The usual glitzy conventions for thousands of party loyalists were reduced to bloodless, almost totally virtual events in August, with Biden supporters watching by video-link as he accepted the Democratic nomination.
The Republican convention was stripped to the bare bones, but Trump was still determined to have his big night, organizing a controversial gala instead at the White House, which is not meant to be used for campaigning.
In a bruising acceptance speech before hundreds of guests, Trump tore into his rival as a threat to the "American Dream."
- Debate chaos -
At the end of September, the two candidates clashed in their first televised duel. It quickly descended into an evening of interruptions, personal attacks and crosstalk that left many viewers shocked.
Both candidates blamed one another and claimed victory.
- Trump catches Covid -
In the early hours of October 2, the president announced that he had tested positive for the coronavirus.
He spent three nights in hospital, declaring after his return to the White House that he had beaten Covid, and calling on Americans not to let the virus "dominate" their lives.
The candidates' second debate was canceled, but the one on October 23 was a much more civilized affair than their bad-tempered first showdown.
- Final hectic push -
Only just recovered, Trump has held a series of packed rallies across swing states right up to polling day.
Many in the audience have not worn masks as the president vowed to secure an against-the-odds win.
Biden took a starkly different approach, always wearing a mask and holding a limited number of small, socially-distanced events.