U.S. Supreme Court Justice Anthony Kennedy announced Wednesday that he will retire from the court next month, providing President Trump the opportunity to ensure a conservative majority on the nation’s highest court.

Justice Kennedy’s retirement, effective July 31, will set up a high-stakes political battle over his replacement.

Serving on the court since 1988, the 81-year-old justice established himself as a sought after swing vote for the court’s liberal justices, casting the deciding vote on issues ranging from abortion and affirmative action to capital punishment and same-sex marriage.

He announced his retirement in a letter to President Trump.

“For a member of the legal profession it is the highest of honors to serve on this court,” he wrote. “Please permit me by this letter to express my profound gratitude for having had the privilege to seek in each case how best to know, interpret and defend the Constitution and the laws that must always conform to its mandates and promises.”

The contest over his successor’s nomination likely will be a tense political battle because of the view of many Democrats that President Trump’s first nominee, Justice Neil Gorsuch, occupies a seat stolen from former President Obama’s nominee, Judge Merrick Garland. Senate Republicans managed to obstruct Judge Garland’s nomination to succeed Justice Antonin Scalia, keeping the seat vacant for 14 months.

Holding a 51-seat majority in the 100-seat U.S. Senate, Republicans ostensibly have the votes to withstand Democratic opposition due to a rule change, advanced last April by GOP Senate Majority Leader Mitch McConnell, which lowered the confirmation threshold for U.S. Supreme Court justices from 60 votes to a simple majority.