Supreme Court Justice Anthony Kennedy, who has often cast the tie-breaking vote between the top US court's liberal and conservative judges, has announced his intention to retire.
Mr Kennedy's departure at the end of July will give President Donald Trump the chance to appoint a more staunchly conservative judge to the nine-seat bench.
That could potentially spell a historic swing to the right for the court, which exerts a deep and far-ranging influence on American life.
"It has been the greatest honour and privilege to serve our nation in the federal judiciary for 43 years, 30 of those on the Supreme Court," Mr Kennedy was quoted as saying in a statement announcing his decision.
Nominated by President Ronald Reagan, the 81-year-old took his oath of office in 1988.
He said his decision to step aside was based on his deep desire to spend more time with his family.
Paying tribute to Mr Kennedy as "a great justice of the Supreme Court," Mr Trump told reporters the replacement process would "begin immediately".
"Hopefully we will pick someone who is just as outstanding," he told reporters in the Oval Office.
Mr Kennedy's vote, positioned at the very centre of the court, gives him a place of special prominence among the nine justices.
He has cast the deciding vote in a number of historic cases, including the high court's groundbreaking decision in 2015 legalising gay marriage, a five-to-four decision for which he wrote the opinion.
For years, he has infuriated conservatives with decisions striking down prayer at public school graduations and upholding abortion rights - and exasperated liberals with decisions on affirmative action and campaign finance laws.
His departure will give Mr Trump his second Supreme Court pick since arriving at the White House, following the nomination of Neil Gorsuch.
Mr Gorsuch was sworn-in in April last year to fill a more than year-long vacancy left by the death of conservative justice Antonin Scalia.
Statement on Justice Anthony Kennedy. #SCOTUS pic.twitter.com/8aWJ6fWemA
— Donald J. Trump (@realDonaldTrump) June 27, 2018
