WASHINGTON - Amid a last dash by candidates courting crucial battleground states, the United States is bracing itself for an election where the outcome looks to be uncertain, possibly even disputed.

Some 24 hours before polls open on Tuesday (Nov 3), Democratic Party candidate Joe Biden remained ahead of his Republican rival Donald Trump across the country.

But in a few battleground states crucial to winning the presidency, the former vice-president's lead is either very slender, or he is in effect in a tie with Mr Trump.

This means Mr Trump could still garner the majority of Electoral College votes to become the winner.

Analysts, however, do not expect election night to go smoothly, especially since millions of mail-in ballots - which are generally seen to favour Democrats - will potentially still be left to count.

This is not unusual; the election night result is normally called on the basis of trends and projections that make it obvious - not on a final official count. But this time, the volume of mail-in ballots appears to be higher than in previous years.

At least 92 million people have already cast their ballots in early voting - nearly twice as many as those who voted early in 2016. Some 230 million Americans are eligible to vote in the presidential election.

For months, the President has been criticising mail-in balloting as prone to fraud - which experts say there is little or no evidence of.

"We should know the result of the election on Nov 3. The evening of Nov 3," Mr Trump insisted on Sunday. "That's the way it's been, and that's the way it should be."

On Sunday, Axios reported that the President had told confidants he would seize the opportunity to declare victory on Tuesday night as results come in, at a point when he is ahead of Mr Biden.

Mr Trump denied that, but said: "I think it's a terrible thing when ballots can be collected after an election. I think it's a terrible thing when states are allowed to tabulate ballots for a long period of time after the election is over."

And signalling protracted legal battles and days of uncertainty, he added: "We're going to go in the night of, as soon as that election's over, we're going in with our lawyers."

This election looks different from 2016, when Mr Trump also trailed his erstwhile rival Hillary Clinton but won the Electoral College by winning a handful of states by less than 100,000 votes all told, the polling organisation Morning Consult said on Monday morning. Mr Biden's lead is 8 percentage points compared with Mrs Clinton's three points in 2016.

Mr Nate Silver, founder of the political analytics website Five Thirty Eight, in a Monday morning note, however, said: "Projected margins in the tipping-point states are considerably tighter than the margins in the national popular vote."

"More specifically, Joe Biden's lead in Pennsylvania - the most likely tipping-point state, according to our forecast - is solid but not spectacular: about 5 points in our polling average."

Pennsylvania has 20 votes in the Electoral College.

"Without Pennsylvania, Biden does have some paths to victory, but there's no one alternative state he can feel especially secure about," Mr Silver wrote.

President Trump was due to hold five rallies in four states in the final hours of the campaign: North Carolina, Pennsylvania, Michigan and Wisconsin.

He planned to wrap up in Grand Rapids, Michigan - the same place he concluded his 2016 campaign.

Mr Biden and his running mate Kamala Harris were due to focus on Pennsylvania and Ohio, with Mr Biden winding up with an evening drive-in rally in Pittsburgh with pop icon Lady Gaga. Pennsylvania, where Mr Biden will rally union members and African American voters in the Pittsburgh area, has become critical to the election.

Meanwhile, fears of unrest and violence, especially if the outcome of the election is contested, have been rising.

Over the weekend in New Jersey, a caravan of Trump supporters blocked a freeway.

In Texas, Trump supporters on a freeway swarmed a Biden-Harris campaign bus; the Democratic Party cancelled events out of safety concerns. The President tweeted: "In my opinion, these patriots did nothing wrong."

In Washington, DC, the White House perimeter is being fortified with a tall fence, and shops and establishments in the surrounding area have been boarded up. Some 250 National Guard troops have reportedly been put on standby.

For live updates and results, follow our US election live coverage.