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.
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 on Friday, two states that may prove crucial on Tuesday.
Both candidates are due to campaign in Florida today. The state is seen as a key contest that could tip the election.
In Cleveland, Ohio, Mrs Clinton ended the day's campaigning at a concert, where she was joined by the singer Beyonce and her husband, rapper Jay Z.
"We have unfinished work to do, more barriers to break, and with your help, a glass ceiling to crack once and for all," Mrs Clinton said.
Addressing the crowd, Jay Z explained his support for Mrs Clinton. He said that though he did not have any "ill will" towards Mr Trump, the Republican's conversation was "divisive".
"That's not an evolved soul to me, so he cannot be my president. He cannot be our president," he said.
The free concert was part of a series of events put on by Mrs Clinton's campaign as she aims to encourage greater African-American participation in the election.
Mr Trump, meanwhile, told a crowd of supporters in New Hampshire 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.
Earlier, at a rally in Pittsburgh, Pennsylvania, Mrs Clinton said that her opponent was "unfit" for office because of his temperament and disparaging comments about women and minorities.
In other campaign developments:
Documents alleging to show that Mr Trump's wife, Melania, worked illegally as a model in the US for a brief period two decades ago before she obtained the legal right to work have been obtained by the Associated Press.
The National Enquirer, a US tabloid, paid $150,000 (£120,000) to a former Playboy model for exclusive rights to the story of an alleged affair she had with Donald Trump from 2006-2007, the Wall Street Journal reports. But the Enquirer, which supports Mr Trump, then killed the story, it says. The National Enquirer has denied paying to suppress the story and the Trump campaign says the alleged affair did not occur.
Latinos are voting early in much greater numbers than at the previous election in key states including Florida, Arizona and Nevada, several reports based on early voting data suggest.
Mr Trump is currently ahead in Ohio, according to a state polling average by Real Clear Politics (RCP), while Mrs Clinton leads by a small margin in Pennsylvania.
In Florida, RCP's poll average puts the Democratic candidate ahead, but poll analysis website FiveThirtyEight says Mr Trump has a 52.4% chance of winning the state's 29 electoral votes.
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.
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. Two senior Democrats have now called for an investigation into the role of former New York mayor Rudy Giuliani, a Trump surrogate.
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.
But Mr Giuliani later denied having prior knowledge of the inquiry, telling CNN he had not received advance notice: "No, I've spoken to no current FBI agents."