In a 5-4 decision an hour before the execution, the majority wrote, “We have no authority to stay an execution in light of an ‘appeal of the President,’ presenting free-ranging assertions of foreign policy consequences, when those assertions come unaccompanied by a persuasive legal claim.”

Kagan, 50, served as the Dean of Harvard Law School from 2003 to 2009. Obama nominated her to serve in her current post as solicitor general early in 2009, and she won Senate confirmation by a vote of 61-31. She is the first woman to serve as solicitor general of the United States.

In a written statement, Chief Justice John Roberts said Stevens “has earned the gratitude and admiration of the American people for his nearly 40 years of distinguished service to the Judiciary, including more than 34 years on the Supreme Court.”

“He has enriched the lives of everyone at the Court through his intellect, independence, and warm grace. We will miss John’s presence in our daily work, but will take joy in his and Maryan’s continued friendship in the years ahead,” Roberts said, referring to Stevens’ wife.

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

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.

The timing of his announcement leaves ample time for the White House to settle on a successor and Senate Democrats, who control 59 votes, to conduct confirmation hearings and a vote. Republicans have not ruled out an attempt to delay confirmation.

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

In a Fox News opinion poll this week, 52 percent of respondents said Obama’s next Supreme Court nominee should be a conservative compared with 29 percent favoring a liberal.

Sen. Patrick Leahy, D-Vt., chairman of the Judiciary Committee, told The Associated Press that midsummer hearings will offer the best chance to avoid partisan fighting over Stevens’ replacement.

In a written statement, he said he hopes the upcoming confirmation battle will produce a “thoughtful and civil discourse.”

“The decisions of the Supreme Court are often made by only five individuals, but they impact the daily lives of each and every American,” he said in a written statement. “All senators should strive to fulfill their constitutional duty of advise and consent, and give fair and thorough consideration to Justice Stevens’ successor.”

Sen. Orrin Hatch, R-Utah, a member of the Judiciary Committee, said the president “has an obligation to nominate judges who understand and are committed to their proper role in our system of government.”

“As I have said for many years, someone who would be an activist judge, who would substitute their own views for what the law requires, is not qualified to serve on the federal bench,” he said in a written statement. “The confirmation process should be fair and thorough, and the president’s nominee should be judged by this standard.”