Milwaukee County Sheriff David A. Clarke Jr. — the controversial, Stetson-wearing official who rose to national prominence with his no-holds-barred conservative rhetoric — resigned his office Thursday.

Clarke, who is in his fourth term, submitted a resignation letter to Milwaukee County Clerk George Christenson on Thursday.

"After almost forty years serving the great people of Milwaukee County, I have chosen to retire to pursue other opportunities," Clarke said in a statement.

Craig Peterson, an adviser to Clarke, said the sheriff would make an announcement about his future next week. "The sheriff wants people to know that he appreciates his 15 years of service and the support he's gotten from the residents of Milwaukee," Peterson said.

Politico reported Thursday that Clarke is expected to take a job in President Donald Trump's administration. But sources close to the sheriff disputed that, saying Clarke is looking at opportunities outside of government that support the Trump agenda and keep Clarke in the public eye.

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Earlier this year, he was passed over for a job in the U.S. Department of Homeland Security. Clarke, a frequent surrogate for Trump during his successful 2016 campaign, also interviewed last year for a job in Trump's cabinet. On Sunday, Trump tweeted favorable remarks about Clarke's memoir.

Gov. Scott Walker will appoint Clarke's successor, but the process won't begin until the governor's office receives the sheriff's resignation letter.

Once that happens, the governor will seek applications, a process that usually takes a couple of weeks, and then begin interviewing candidates. Applicants must live in Milwaukee County to be appointed to the post.

Clarke's successor will serve until the end of the current term in 2018.

For now, Inspector Richard Schmidt will serve as acting sheriff.

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Milwaukee County Executive Chris Abele, who has had a fractured relationship with Clarke, said he was given no notice of his resignation. Abele said this will give the county the opportunity to find a sheriff who is "more interested in integrating with the rest of the community and maybe more focused on solutions and allies than enemies and fights."

Clarke rose to national prominence beginning with his narrow 2014 Democratic primary victory, overcoming heavy outside spending by former New York Mayor Michael Bloomberg with the help of the National Rifle Association.

Shortly after, Clarke became the go-to law enforcement official to defend police officers as they faced protesters after shootings in Ferguson, Mo., New York City and elsewhere. He was particularly critical of the Black Lives Matter movement, which he dubbed "Black Lies Matter."

Clarke parlayed his newfound prominence into a lucrative side career, giving lectures and talks at conservative conferences, conventions and cruise boats. In 2016, he collected $220,172 in speaking fees, gifts, lodging, airfare and other travel-related expenses while speaking to 34 different groups.

Clarke's many critics greeted his resignation letter with glee.

“I want to thank Sheriff Clarke for his decision to step down," said state Sen. Lena Taylor (D-Milwaukee). "After years of abuse at his hands, the people of Milwaukee can sleep soundly tonight.”

But Clarke had his share of supporters, as shown by his long tenure as sheriff.

County Supervisor Dan Sebring, a longtime and vocal supporter of Clarke, said that the former sheriff "has done a great deal of good for Milwaukee County."

Supervisor Deanna Alexander said: "News of Sheriff Clarke's retirement comes as a surprise. However, I wish him well in whatever his next endeavor is, whether that is leading another government agency or a pursuing a restful retirement. David Clarke ... has served Milwaukee County honorably and I am sad to see him go."

The son of a Korean war veteran, Clarke — a lifelong Milwaukee resident — has been in law enforcement through most of his adult life. He joined the Milwaukee Police Department in 1978, serving as a patrolman for 11 years and rising to captain in 1996.

He was catapulted into the job of Milwaukee County sheriff after the resignation of Lev Baldwin. Ten people applied for the opening, and then-Gov. Scott McCallum, a Republican, appointed Clarke in March 2002.

At the outset of his tenure, Clarke was hailed as a fresh face who could bring new ideas and energy to the department. He earned support from conservatives and liberals, Republicans and Democrats.

But over the years, as his rhetoric grew harsher, he moved steadily to the right. Even while continuing to run as a Democrat, he relied on a conservative base to win re-election.

The list of people interested in replacing Clarke includes some heavy-hitters.

Former Milwaukee Police Capt. Earnell Lucas, now a vice president of security with Major League Baseball, registered as a candidate for sheriff in 2018 and has been campaigning for months.

Lucas confirmed Thursday he will seek an appointment to the post.

Also, U.S. Marshal Kevin Carr, who served as Clarke's top deputy for several years, has said he would seek Walker's appointment if the sheriff resigned. He was not immediately available for comment.

Milwaukee County Judge John Siefert has said he plans to run as a Democrat for sheriff next year. But, unlike Lucas, he has said he would not seek an appointment from Walker.

Clarke is leaving behind many unresolved issues in Milwaukee County.

Most notably, he has come under fire for the deaths of at least five individuals at the Milwaukee County Jail since 2016.

In May, prosecutors ordered a public inquest into the death of Terrill Thomas, 38, a mentally ill man who died of dehydration in a solitary cell in 2016, one of four deaths in the jail from April to November last year. The jury in that inquest found probable cause that seven jail officials committed criminal abuse, neglect or ill-treatment of inmates.

Milwaukee County District Attorney John Chisholm has not decided whether to file charges.

In addition, two federal lawsuits have been filed against Clarke and others over Thomas's death. Clarke is also the subject of another federal suit over the death of an infant who was born while her mother was an inmate being held alone in a cell.

Clarke has embroiled in other litigation as well.

A man who shared a commercial flight with Clarke sued him after he claims Clarke had him detained and questioned upon landing in Milwaukee, after the man made a comment to Clarke about wearing Dallas Cowboys gear. That federal civil rights case is set for trial in January.

Bruce Vielmetti, Ashley Luthern and Don Behm of the Journal Sentinel staff contributed to this report.