Forty-five percent of registered voters in the Sept. 10-14 survey said they would choose Biden if the election were held today, a 2 percentage point decrease from the previous poll conducted earlier this month.
By contrast, 39 percent of voters said they'd cast their ballots for Trump, no change from the last survey.
Four percent of voters said they'd prefer someone else as their presidential candidate, a 1 percentage point dip, while another 4 percent said they do not plan to vote.
Eight percent of voters said they were still unsure, a 1 percentage point tick up.
The survey found Biden's lead among senior voters, ages 65 and older, fell 9 percentage points, from a 10 point lead in the Sept. 5-8 poll to a 1 point lead in this most recent study.
The Hill-HarrisX poll was conducted online among 3,758 registered voters between Sept. 10 and 14. It has a margin of error of plus or minus 1.6 percentage points.