Registered voters back the former vice president over the incumbent 52 - 37 per cent, according to a Quinnipiac University national poll released on Wednesday. The same poll one month ago gave Mr Biden a lead of eight points, 49 - 41 per cent.
The poll suggested that independent voters were behind the shift. Back in June, independent voters were split nearly evenly between the two — now Mr Biden leads 51 - 34 per cent.
"Yes, there's still 16 weeks until Election Day, but this is a very unpleasant real time look at what the future could be for President Trump. There is no upside, no silver lining, no encouraging trend hidden somewhere in this survey for the president," said Quinnipiac University polling analyst Tim Malloy.
The poll surveyed 1,273 self-identified registered voters nationwide between July 9 - 13, with a margin of error of 2.8 percentage points.