Twenty-nine percent of Democrats and independents said they would vote for Biden, up from 24 percent who said so in a poll that ran in late April, days before Biden announced his bid, according to the latest poll released on Wednesday.
Biden led the field among all major demographic groups except those ages 18-37, who favoured U.S. Senator Bernie Sanders of Vermont over Biden 18 percent to 16 percent.
In terms of overall support rate, 13 percent of Democrats and independents said they would vote for Sanders.
None of the other candidates received more than 6 percent support in the poll. Biden, 76, was the U.S. vice president from 2009 to 2017 after representing Delaware in the Senate from 1973 to 2009.
The Reuters/Ipsos poll was conducted online in English throughout the United States on May 10-14 over 1,132 Democrats and independents.
Respondents were asked to pick from 23 potential Democratic nominees, including New York City Mayor Bill de Blasio, who is expected to jump into the race on Thursday.