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, two states that may prove crucial on Tuesday.
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 the Democratic presidential candidate’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.