More than 22m Americans had voted early by Friday, either in person or by mail, according to the US Election Project.
At the same point in the 2016 race, about 6m votes had been cast.
Experts say the surge in early voting correlates to the coronavirus pandemic, which has caused many people to seek alternatives to election day voting.
Why we might not get a result on election night
On Tuesday, Texas, a state that has relatively tight restrictions on who can qualify for postal voting, set a record for most ballots cast on the first day of early voting.
On Monday, the Columbus Day federal holiday, officials in Georgia reported126,876 votes cast – also a state record.
Reports indicate that registered Democrats have so far outvoted registered Republicans – casting more than double the number of ballots. And of these early voting Democrats, women and black Americans are voting in particularly high numbers. Some are motivated by dislike for Donald Trump, while others have been energised by racial justice protests throughout the summer following the police killing of George Floyd in Minnesota.
World reacts to long queues of voters in US
What is US row over postal service about?
But this early advantage does not mean that Democrats can already claim victory. Republicans, who claim postal voting is vulnerable to fraud, say Democrats may win the early vote, but that Republicans will show up in large numbers on election day.
According to a 2017 study by the Brennan Center for Justice, the rate of voting fraud overall in the US is between 0.00004% and 0.0009%.
The enormous numbers of voters have led to long lines, with some people waiting for up to 11 hours for an opportunity to vote.