Stevens, one of the oldest and longest-serving justices in history, sent a letter to the White House notifying Obama of his retirement this summer. Stevens, who turns 90 in 11 days, joined the court in 1975 after being appointed by President Gerald Ford.

A confirmation battle could sidetrack Democratic plans to focus on the economy and job creation ahead of November's congressional elections in which Republicans are hoping to regain control of Congress.

Democrats reacting to Stevens' announcement were pressing for a quick confirmation of any nominee while Republicans were promising thorough scrutiny.

The departure of Stevens means Obama will make his second Supreme Court appointment. Last year, he named US appeals court judge Sonia Sotomayor of New York as the Supreme Court's first Hispanic in filling the vacancy created by Justice David Souter's retirement.

Stevens said in his letter that he would retire one day after the Supreme Court's current term ends, which is expected in late June.

Stevens said he had concluded it would be in the best interests of the court to have his successor appointed and confirmed by the Senate well in advance of the start of the court's next term, which begins in October.

Obama is expected to nominate a liberal to replace Stevens and then push hard to win the required Senate confirmation for the lifetime appointment. It would not change the court's overall ideological balance, divided with five conservatives and four liberals.

Obama administration officials and legal experts said the leading candidates to replace Stevens were expected to be current Solicitor General Elena Kagan and US appeals court judges Diane Wood and Merrick Garland.

The confirmation battle could dominate Congress for some time and make it tougher for Obama's fellow Democrats to focus on reducing the US unemployment rate, expected to be a key issue in the congressional elections.

It also could complicate last-ditch efforts by some Democrats and Republicans to win Senate passage of compromise legislation to combat global climate change.

Potential nominees Kagan, Wood and Garland, all considered moderate liberals, could face varying degrees of Republican opposition. But even conservative activists said each probably would win confirmation in a Senate in which Democrats control 59 of 100 seats. A simple majority is needed for confirmation.

US Senate Judiciary Committee Chairman Patrick Leahy, who will chair the confirmation hearing for Obama's nominee, said he expected the president to consult with members of both political parties before making his pick.

"I hope that senators on both sides of the aisle will make this process a thoughtful and civil discourse," Leahy said.

Senate Republican Leader Mitch McConnell made it clear that his party would be heard this election year in Obama's bid to win confirmation of a successor to Stevens.

"As we await the president's nominee to replace Justice Stevens ... Americans can expect Senate Republicans to make a sustained and vigorous case for judicial restraint and the fundamental importance of an even-handed reading of the law," McConnell said.

Supreme Court appointments have become major political battles in Congress. The high court decides contentious social issues such as abortion and the death penalty and high-stakes business disputes.

There was no immediate indication on when the confirmation hearings would be held but it likely will be before Congress departs for its August recess in advance of the November congressional elections.