Biden is in the lead with 31 percent support, followed by Sen. Bernie Sanders (I-Vt.) at 19 percent, and Sen. Kamala Harris (Calif.) at 14 percent, according to a poll released on Tuesday by Morning Consult, Presstv Reported.
In the past three weeks, Biden's loss of points comes as Harris has improved by 8 points in that time.
At the first Democratic debate in late June, Harris hammered Biden over remarks he made about having found common ground working with segregationists in the Senate decades ago. And she challenged Biden over his past opposition to a federal busing law aimed at integrating schools.
Biden, for his part, defended his record on racial issues and reminded voters of his ties to former President Barack Obama, whose popularity in the US remains high.
In the meantime in a possible face-off with Trump, Biden remains the top choice among black voters, with 38 percent support.
Sanders is second place with 21 percent, while Harris is third with 16 percent support among black voters.
The Morning Consult online survey of 16,599 potential Democratic primary voters was conducted between July 1 and July 7 and has a 1 percentage point margin of error.