Four years ago at this stage of the campaign, more than twice as many people were similarly wavering between Trump and his Democratic opponent, Hillary Clinton.
The Reuters/Ipsos polling shows Biden holding a 10-percentage-point lead nationally, with a tighter margin in the battleground states that will help decide the election.
Democratic fundraising organization ActBlue said on Thursday it collected $1.5 billion US online from July to September, the most it had ever raised in one quarter.
Both candidates have been visiting battleground states this week, with Trump holding rallies in Florida, Pennsylvania and Iowa and Biden traveling to Ohio and Florida.
Speaking to a rally in Greenville, N.C., on Thursday, Trump promised an economic recovery if he was re-elected.
ET, with Trump on NBC from Miami and Biden on ABC from Philadelphia.
A group of 100 Hollywood actors and producers wrote a letter of protest to NBC, saying that airing Trump's town hall was "enabling the president's bad behaviour while undercutting the Presidential Debate Commission and doing a disservice to the American public.".
The Biden campaign said on Thursday that two people involved in the campaign had tested positive for COVID-19, including one on the staff of Sen. Kamala Harris, Biden's running mate.