Minnesota Supreme Court Justice David Lillehaug, citing early stage Parkinson's disease, said Wednesday that he will retire from the bench next year, likely setting up the first high-court opening to be filled by new DFL Gov. Tim Walz.

The 65-year-old Lillehaug, a former Minnesota U.S. attorney and DFL candidate for the U.S. Senate, said he initially intended to seek re-election in 2020 but instead decided to leave the court before the end of his term.

His planned retirement on July 31, 2020, will cap a decadeslong career in Minnesota law and politics, where he forged close ties to some of the state's most high-powered Democratic figures, from former Vice President Walter Mondale and the late Sen. Paul Wellstone to former Gov. Mark Dayton, who appointed him to the high court in 2013.

In an interview Wednesday, Lillehaug said he first noticed a twitch in his thumb last fall and received a tentative Parkinson's disease diagnosis in November that was confirmed earlier this year. He said he opted to set a retirement date after talks with his wife and daughter, calling it a difficult choice to think about leaving what he called "the greatest honor of my life."

"It's the same thing that is both wonderful and a little frightening," Lillehaug said of his judicial role. "Which is we are the last word on the law for the state of Minnesota. It is the responsibility to take care of the law in Minnesota that I'll most miss."

Lillehaug, who served as U.S. attorney for Minnesota under President Bill Clinton, also had a long career in private practice advising and litigating cases for the DFL Party, including work on the recount battles that helped elect Dayton in 2010 and former Sen. Al Franken in 2008.

His Parkinson's disease diagnosis follows a bout with throat cancer in 2013.

Walz said he spoke with Lillehaug about his decision on Tuesday evening and called him "a person who spent a lifetime of serving Minnesota, serving this country."

"A sterling reputation as a jurist, and someone who will be deeply missed on the court," Walz said. "I think, knowing David, and where he's at, he's making this decision to make the most out of his life, spend time with his family."

Lillehaug left open his plans for after he leaves the court, saying he recognizes "an uncertain window of opportunity" for post-retirement travel and outdoor activities with family. He added that he also plans to work part-time but said it was too early to tell what he would do.

Although Lillehaug's seat would be up for re-election in 2020, his departure triggers a process by which Walz will have to fill the vacancy with a justice who would serve two years before being on a statewide ballot. Walz told reporters that he believed it was important to fill that seat early for the continuity of the court, adding that voters would have their say in 2022.

As Walz's predecessor, Dayton reshaped the state's courts through eight years of appointments that dramatically diversified Minnesota's crop of jurists at all levels. Walz has kept up the tradition of using a Commission on Judicial Selection to review lower-court openings. The 49-member commission vets candidates and sends the governor three top finalists to pick from. Walz called the process "the best way to put the most folks forward."

"I think … using the constitutional authority that I have in this office and the capacity to bring a thoughtful approach to who we put there, and then put that person in front of the voters to make that case — I think that's the appropriate way to do it," Walz said.

Lillehaug was named U.S. attorney for Minnesota — making him the top federal law enforcement officer in the state — in 1994 on Wellstone's recommendation. He later ran for state attorney general and the U.S. Senate. He was appointed to the Minnesota high court in 2013 to succeed Paul Anderson, who retired on reaching the mandatory retirement age of 70. He then retained his seat on the bench in an election in 2014.

As U.S. attorney in the 1990s, he took on high-profile criminal cases against Qubilah Shabazz, Malcolm X's daughter, and famed surgeon John Najarian. He also shaped the office's criminal prosecutions of gang leaders and white-collar criminals, including a case that ended with a former developer receiving one of the longest sentences imposed in the country at the time.

Lillehaug, a South Dakota native, is a graduate of Augustana College and received his law degree from Harvard. From the beginning, Lillehaug was assertive about his lofty political ambitions with a sharp intelligence and manner.

"He was direct and aggressive and let you know his thoughts," said John Koneck, president of the Minneapolis-based Fredrikson & Byron, where Lillehaug worked before his appointment to the bench. "You didn't find a more ethical, principled lawyer that handles pressure well."

In private practice, Lillehaug also served as special counsel for Dayton during the 2011 government shutdown and worked for the DFL during redistricting in 2012.

"He brought what he always brought to every matter he worked on: a tremendous focus to the issues, a work ethic that cannot be topped, and a brilliant view on the issues that are a result of the fact that he is a brilliant guy," said Charles Nauen, a Minneapolis attorney who practiced alongside Lillehaug in the highly partisan recount cases.

But in the past six years, Lillehaug has sought to cloak his partisan past with the black robe that hangs in his St. Paul chambers.

"One's life changes when you raise your right hand and swear an oath to the Constitution," Lillehaug said. "You leave politics and political personalities behind. It's about precedent."

Earlier this year, Lillehaug took the lead on a Supreme Court initiative to curb high levels of depression, anxiety and stress in the legal profession, following the death of a good friend who had taken his own life after suffering from untreated depression while working in private practice in the 1990s.

"We added some fuel to the fire for lawyer well-being," Lillehaug said.

On Wednesday, the justice was not yet prepared to look back, noting the more than a year of deliberation still ahead of him alongside six colleagues with whom he has grown close.

"The most important case has to be the one I heard this morning," said Lillehaug, speaking moments after oral arguments in a court challenge over the city of Bloomington's garbage collection. "Every case is the most important one."

Staff writers Jessie Van Berkel, Rochelle Olson and Paul Walsh contributed to this report.

Twitter: @smontemayor