Vice President Kamala Harris’s efforts to showcase her campaign’s political embrace of Republicans have led to the endorsements of archconservatives like Liz Cheney and onetime Tea Party darlings like Adam Kinzinger.

Now, Ms. Harris can boast the backing of a well-known former right-wing firebrand who amplified false claims of voter fraud and once referred to Michelle Obama in racist terms.

Charlie Sykes, the former conservative talk-radio host, is scheduled to moderate a conversation with Ms. Harris and Ms. Cheney on Monday in the Milwaukee suburb of Brookfield. He has undertaken a political transformation that is perhaps greater than either Ms. Cheney’s or Mr. Kinzinger’s. During his heyday on Milwaukee radio, he regularly suggested shadowy outside forces were engaging in voter fraud to rig local elections and on occasion referred to Mrs. Obama using the racist sobriquet “Mooch.” In 2009, he emceed an event with Sarah Palin for the anti-abortion group Wisconsin Right to Life.

Until 2016, when he emerged as a leading conservative critic of Donald J. Trump’s first presidential bid, Mr. Sykes was a reliable mouthpiece for right-wing causes in Wisconsin. He was instrumental in the political rise of Senator Ron Johnson and Gov. Scott Walker, Republicans he later broke with after turning on Mr. Trump.

More recently, Mr. Sykes wrote a book denouncing Trump-allied conservatives and became an explainer of the political right on MSNBC and the center-right outlet The Bulwark, which he founded in 2018 and left in February. In 2021, Mr. Sykes announced he was no longer a Republican. This month, he said he would vote for Ms. Harris.

“There are a lot of things that I’m embarrassed about and now regret and for which I have apologized,” Mr. Sykes said on Saturday. “I wrote a book, ‘How the Right Lost its Mind,’ and have spent more than a few years trying to atone for a lot of that.”