Former Vice President Joseph R. Biden leads President Trump by 9 points in a hypothetical match-up in the all-important state of Wisconsin, according to a survey released Wednesday.

Mr. Biden held a 9-point, 51% to 42% lead over Mr. Trump, according to the Marquette University Law School poll.

Sen. Bernard Sanders of Vermont led Mr. Trump by 4 points, 48% to 44%. Sen. Elizabeth Warren of Massachusetts and Mr. Trump were tied at 45% apiece, and Sen. Kamala D. Harris of California and Mr. Trump were tied at 44% apiece.

Overall, 45% of respondents said they approve of the job Mr. Trump is doing, compared to 53% who said they disapprove.

Mr. Trump’s narrow win over Democratic presidential rival Hillary Clinton in Wisconsin in 2016 was crucial to his overall victory, as the state had not broken for a Republican presidential candidate since 1984.

Among potential Democratic primary voters, Mr. Biden was the top choice of 28% of respondents, followed by Mr. Sanders at 20%, Ms. Warren at 17%, Pete Buttigieg, mayor of South Bend, Indiana, at 6%, and Ms. Harris at 3%.

The overall survey of 800 registered voters was conducted from Aug. 25-29 and has a margin of error of plus or minus 3.9 percentage points.

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