Mr Biden campaigned in Pennsylvania and Ohio, as Mr Trump toured the voting battlegrounds of Wisconsin, Michigan, North Carolina and Pennsylvania.
National polls suggest a firm lead for Mr Biden in Tuesday's election.
But his lead is narrower in the handful of states that could decide the result.
More than 98 million people have already cast their ballots in early voting, putting the country on course for its highest turnout in a century.
In the US election, voters decide state-level contests rather than an overall single national one.
To be elected president, a candidate must win at least 270 votes in what is called the electoral college. Each US state gets a certain number of votes partly based on its population and there are a total of 538 up for grabs.
This system explains why it is possible for a candidate to win the most votes nationally - like Hillary Clinton did in 2016 - but still lose the election.
Tuesday's vote comes amid the ongoing coronavirus pandemic. The US has recorded more cases and more deaths than any other country worldwide, reporting more than 81,000 new infections on Sunday alone.
Top virus expert Anthony Fauci has sharply criticised the Trump administration's handling of the pandemic, drawing a rebuke from the White House on Sunday.
Democrats candidate Joe Biden made a last-minute campaign stop for a drive-in rally in Ohio