So far nearly 26 million people have cast a ballot in the election, shattering records, according to the US Elections Project, run by the University of Florida.
While Trump lags in opinion polls at a national level and in many battleground states, Biden campaign manager Jen O'Malley Dillon said over the weekend that the national figures are misleading because must-win states are close.
"We cannot become complacent because the very searing truth is that Donald Trump can still win this race, and every indication we have shows that this thing is going to come down to the wire," she wrote in a memo to donors.
US presidential elections are determined by electoral votes, allotted to US states and territories based largely on their populations, rather than by a tally of the popular vote nationwide. This makes for a close focus by campaigns on states that could go either way.
Trump campaigned on Saturday in Michigan and Wisconsin, two battleground states he narrowly won in the 2016 election.
In Muskegeon, Mich., he attacked Michigan Governor Gretchen Whitmer, a Democrat, blasting state rules to stop the spread of coronavirus, calling her “dishonest,” and making light of a right-wing plot uncovered by the FBI to kidnap her.