MADISON, Wis. — Outspoken Milwaukee County Sheriff David Clarke, who built a following among conservatives nationally with his provocative social media presence and strong support of Donald Trump, resigned on Thursday.

Milwaukee County Clerk George Christenson said he received a resignation letter from Clarke, but the letter did not say why the sheriff was leaving office more than a year before his term is up.

Clarke’s office referred questions to an email address that he had provided. He did not immediately respond to an email seeking his reasons for leaving or what he planned to do next.

Clarke’s most recent Twitter post from Thursday afternoon showed him posing with law enforcement officers at the National Fraternal Order of Police convention in Nashville.

Some Wisconsin conservatives had encouraged Clarke to challenge U.S. Sen. Tammy Baldwin, a Democrat, but he repeatedly rejected their overtures.

Clarke announced in May that he had taken a job at the Department of Homeland Security, but the agency never confirmed it. He later said he withdrew his name. He recently published a memoir, “Cop Under Fire.”

The tough-talking, cowboy hat-wearing firebrand made himself a darling of the political right through his brash social media presence, his staunch support for Trump’s crackdown on illegal immigration and his support for patrolling Muslim neighborhoods.

Clarke was one of the few African-Americans to speak at the Republican National Convention last year. He has been vocal about gun rights and critical of what he called the “hateful ideology” of the Black Lives Matters movement, saying at times, “Stop trying to fix the police. Fix the ghetto.”

Clark has been sheriff of Milwaukee County since 2002 and spent more than two decades before that with the city’s police department.

A lengthy inquest into the dehydration death of a Milwaukee County Jail inmate earlier this year raised troubling questions about how Clarke managed the jail — just as the White House was said to be considering him. Despite that investigation, Clarke remained popular with many conservatives.

The frequent Fox News guest earned more than $105,000 last year in speaking fees — almost as much as his sheriff’s salary — at more than three dozen events across the country.

Republican Gov. Scott Walker would be responsible for appointing someone to serve the remainder of Clarke’s term, which runs through 2018. Walker’s spokesman Tom Evenson said once he receives official notification of his resignation, the process of finding a replacement will begin.

Evenson did not immediately respond to an email seeking reaction from the governor to Clarke’s decision to resign.