Former Vice President Joe Biden topped President Trump by 9 percentage points in a Fox News poll released on Saturday.

The likely Democratic nominee was backed by 49 percent of the survey's respondents in a theoretical matchup against Trump, who had 40 percent support. An additional 11 percent said they are undecided or will vote for someone else.

Last month, Biden had an 8-point lead over Trump, at 49 to 41 percent.

The former vice president in the new poll has a 9-point lead over Trump among voters who are extremely interested in the election, 52 to 43 percent. Biden also holds an 8-point advantage in battleground states, 48 to 40 percent.

But in counties where former Secretary of State Hillary Clinton and Trump were within 10 points of each other in 2016, Biden has a 25-point advantage, 57 to 32 percent.

When the theoretical matchup included Biden's potential vice presidential candidate, only Sen. Elizabeth Warren (D-Mass.) tipped the Democratic ticket to majority support at 52 percent, compared to the Trump ticket at 42 percent.

Sen. Kamala Harris (D-Calif.) or Sen. Amy Klobuchar (D-Minn.) both push the Biden ticket to 50 percent, while Trump's ticket stays at 42 percent.

Biden has committed to choosing a woman as his vice presidential candidate.

The Fox News poll of 1,011 randomly chosen registered voters was conducted between March 21 and 24. It has a margin of error of 3 percentage points.