As opinion polls suggest Donald Trump is gaining support, Hillary Clinton has been holding rallies in states that had been considered safe for the Democrats.
A BBC correspondent says both teams are now concentrating more on getting their supporters to vote rather than swaying those undecided.
Thirty-seven million early ballots have already been cast.
US authorities say they are assessing the credibility of information on a possible al-Qaeda terror attack.
New York, Texas and Virginia are believed to be the potential targets mentioned in connection with a possible attack before election day on Tuesday, but a police spokesman said the information “lacks specificity”.
Officials say they regularly assess all possible threats before major events.
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News of a possible attack came as both Mrs Clinton and Mr Trump made final pushes for support in battleground states across the US.
They both held rallies in Ohio and Pennsylvania, two states that may prove crucial on Tuesday.
Mr Trump was also in New Hampshire, where he told a crowd of supporters that his rival wants a “550% increase” on Syrian refugees allowed into the US.
“Her plan would mean generations of terrorism, extremism, and radicalism spreading into our schools and communities,” the Republican candidate said.
Mrs Clinton, meanwhile, told a rally in Pittsburgh, Pennsylvania, that her opponent was “unfit” for office because of his temperament and disparaging comments about women and minorities.
The Democrats’ campaign chief, John Podesta, said Mr Trump should ask the governor of New Jersey, Chris Christie, to step down as the head of his transition team after two of his aides were found guilty of orchestrating traffic jams in New Jersey as an act of political retribution against a local Democratic mayor.
Mr Christie has denied any involvement.
Mr Trump is currently ahead in Ohio, according to a state polling average by Real Clear Politics, while Mrs Clinton leads by a small margin in Pennsylvania.
National polls have suggested that the Republican candidate has gained substantial ground on his rival in the last week or so. That momentum also appears to have helped him in several key battleground states.
Who is ahead in the polls?
The Democratic nominee has had a tough few days after the FBI said last week it was looking into emails that may be connected to her use of a private email server while she was secretary of state in the Obama administration.
Mrs Clinton has said she is confident the new inquiry will not change the FBI’s original finding in July, which criticised her but cleared her of any illegal acts.
The Clinton camp have questioned the timing of the announcement and two senior Democrats have now called for an investigation into Trump surrogate and former mayor of New York Rudy Giuliani’s role in it.
In an interview on Fox News on Friday morning, Mr Giuliani appeared to suggest he knew about the inquiry before it was announced, saying: “I had no role in it. Did I hear about it? You’re darn right I heard about it.”
Democratic Representatives Elijah Cummings and John Conyers wrote to the Justice Department on Friday afternoon asking for an investigation into the source of the information to Mr Giuliani.
“Leaking this information to former FBI officials as a conduit to the Trump campaign is equally intolerable,” they wrote in the letter.
But Mr Giuliani later denied having prior knowledge of the inquiry, telling CNN he had not received advance notice from FBI: “No, I’ve spoken to no current FBI agents.”
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US election 2016: Clinton seeks to secure Democratic vote