President Barack Obama has saluted retiring Supreme Court Justice John Paul Stevens as an “impartial guardian” of the law, and says he’ll move quickly to nominate a successor.

Obama paid tribute to the 89-year-old Stevens shortly after the president returned from Prague where he signed a new nuclear arms limitation treaty with Russia.

Obama said Stevens was a “brilliant” jurist and he hailed his appointment by former President Gerald Ford in the months after the Watergate scandal.

The president said Stevens had “worn the judicial robe with honor and humility.” Stevens’ decision to step down from the high court bench gives Obama his second Supreme Court appointment.

Stevens said Friday he will step down when the court finishes its work for the summer in late June or early July. He said he hopes his successor is confirmed “well in advance of the commencement of the court’s next term.”

Stevens’ announcement leaves ample time for the White House to settle on a successor and for Senate Democrats, who control a 59-vote majority, to hold confirmation hearings and a vote before the court’s next term begins in October. Republicans have not ruled out attempts to delay confirmation.

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Stevens’ announcement had been hinted at for months. It comes 11 days before his 90th birthday.

Throughout his tenure, which began after President Gerald Ford nominated him in 1975, Stevens usually sided with the court’s liberal bloc in the most contentious cases — those involving abortion, criminal law, civil rights and church-state relations. He led the dissenters as well in the case of Bush v. Gore that sealed President George W. Bush’s election in 2000.

Stevens began signaling a possible retirement last summer when he hired just one of his usual complement of four law clerks for the next court term. He acknowledged in several interviews that he was contemplating stepping down and would certainly do so during Obama’s presidency.

Chief Justice John Roberts said in a written statement that Stevens “has enriched the lives of everyone at the Court through his intellect, independence, and warm grace.”

Senate confirmations of Supreme Court justices have increasingly become political battles and this one will come amid the added heat of congressional election campaigns.

Sen. Patrick Leahy, D-Vt., chairman of the Senate Judiciary Committee, appealed for civility. “I hope that senators on both sides of the aisle will make this process a thoughtful and civil discourse,” Leahy said.

Looking toward those hearings, Senate Republican Leader Mitch McConnell of Kentucky said, “Americans can expect Senate Republicans to make a sustained and vigorous case for judicial restraint and the fundamental importance of an evenhanded reading of the law.”

Stevens informed Obama in a one-paragraph letter addressed to “My dear Mr. President.” It was delivered to the White House by court messenger at 10:30 a.m. EDT, two minutes before the court’s public announcement. The news came on a day when the court wasn’t in session.

White House counsel Bob Bauer telephoned the news to Obama on Air Force One, as he returned from a trip to Prague.

The leading candidates to replace Stevens are Solicitor General Elena Kagan, 49, and federal appellate Judges Merrick Garland, 57, in Washington and Diane Wood, 59, in Chicago.