Biden leads Trump by 45 percent to 39 percent, the results of the poll showed on Tuesday.
The poll showed a softening in Biden's support, marking it two points down the previous Hill-HarrisX poll conducted earlier in September.
Biden's support among voters 65 and older fell by nine points, according to the poll.
Trump's support was holding firm but not convincing any Independent or other potential swing voters to join him, the poll results also showed.
The Hill-HarrisX poll was conducted online among 3,758 registered voters between 10 and 14 September and with a margin of error of plus or minus 1.6 percentage points, The Hill said.