Democratic U.S. Senate candidate Mandela Barnes' campaign is blaming a "clerical error" for listing a La Crosse County Sheriff's Department captain as one of nine endorsements from law enforcement officers when the officer did not, in fact, back Barnes.

Barnes' campaign on Thursday released the list of nine endorsements from current and former law enforcement officers. One of them was John Siegel, who was listed as a police captain for the city of La Crosse. On Monday, the conservative website Wisconsin Right Now reported an interview with Seigel in which he said he never endorsed Barnes.

Barnes spokesperson Maddy McDaniel told Wisconsin Public Radio on Monday that it was a mistake "due to a clerical error," which was corrected on the campaign website Saturday.

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Wisconsin Lieutenant Governor Mandela Barnes, who is running to become the Democratic nominee for the U.S. Senate, speaks during a campaign event at The Wicked Hop in Milwaukee. Barnes' campaign is blaming a "clerical error" for listing a La Crosse County Sheriff's Department captain as one of the endorsements when the officer did not back Barnes. (Scott Olson/Getty Images)

Barnes is running against Republican U.S. Sen. Ron Johnson in a contest that polls show is about even. Johnson campaign spokesman Alec Zimmerman accused Barnes of "lying about who supports him."

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Siegel is a patrol captain with the La Crosse County Sheriff’s Department and running as a Democrat to replace outgoing Sheriff Jeff Wolf. He told WISC-TV he had a brief conversation with a member of the Barnes campaign but said "I didn’t ever agree to put my name on anything or be added to a list."