Sen. Bernard Sanders on Sunday tried to shrug off new polling that shows support for him among the Democratic presidential field has dropped 10 points since March, saying he believes he’s the strongest candidate to take on President Trump head-to-head.

“Well, I explain it in the sense that polls go up and polls go down, but the other part of the poll if I heard you correctly … is that I am beating President Trump fairly decisively, which is consistent with all of the polling that I have seen,” the Vermont independent said on “Fox News Sunday.”

“I think, frankly, I am the strongest candidate to defeat Trump,” Mr. Sanders said. “I think we can win in Pennsylvania, Wisconsin and Michigan and some of the other battleground states and that’s a fight that I look forward to.”

A new Fox News poll released on Sunday showed former Vice President Joseph R. Biden leading the 2020 Democratic field with 32% support among primary voters, followed by Mr. Sanders at 13%, Sen. Elizabeth Warren at 9%, and South Bend Mayor Pete Buttigieg and Sen. Kamala Harris at 8% each.

Mr. Sanders had been at 23% support in a March poll. Since then, support for Ms. Warren has increased by 5 percentage points and support for Mr. Buttigieg increased by 7 points, while support for Mr. Biden and Ms. Harris stayed relatively steady.

To Mr. Sanders’ point, he did lead Mr. Trump by 9 percentage points, 49% to 40%, in a head-to-head match-up — though Mr. Biden held a 10-point lead, 49% to 39%.

The match-ups were closer elsewhere. Ms. Warren held a 2-point lead, 43% to 41%, against Mr. Trump; Ms. Harris was up by 1 point, 42% to 41%; and Mr. Buttigieg was up by 1 point, 41% to 40%.

The survey of 1,001 registered voters, which included 449 Democratic primary voters, was conducted June 9-12. It has a margin of error of plus or minus 3 percentage points for all registered voters and 4.5 points for Democratic primary voters.