Biden's progress this month comes as Congress looked into allegations that President Donald Trump tried to pressure a foreign leader to investigate him, according to a Reuters/Ipsos poll released on Friday.
The October 17-18 opinion poll found that 21 per cent of Democrats and independents said they would vote for Biden in statewide nominating contests that begin next year, AAP reported.
This was up three percentage points from a similar poll that was conducted at the end of September.
Another 16 per cent said they would support US Senator Bernie Sanders of Vermont and 15 per cent said they would back US Senator Elizabeth Warren of Massachusetts.
Mayor Pete Buttigieg received about five per cent support and US Senator Kamala Harris and former Texas congressman Beto O'Rourke both received three per cent.
The race for the Democratic nomination remains wide open.
About one in five said they remain undecided, and nearly two-thirds of Democrats and independents said they could still change their minds.