Published: Apr 09, 2010 9:09 PM EDT

 WASHINGTON (AP) - The retirement of John Paul Stevens, the Supreme Court's leading liberal but a justice who also could find conservative allies, will set off an election-year political battle over President Barack Obama's second high court pick.

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 will be confirmed "well in advance of the commencement of the court's next term." Obama hailed Stevens as an "impartial guardian of the law" and promised to move quickly to nominate a replacement.

"We cannot replace Justice Stevens' experience or wisdom," Obama said at the White House after returning from a nuclear treaty-signing trip to Prague. "I'll seek someone in the coming weeks with similar qualities: an independent mind, a record of excellence and integrity, a fierce dedication to the rule of law and a keen understanding of how the law affects the daily lives of the American people. It will also be someone who, like Justice Stevens, knows that in democracy powerful interests must not be allowed to drown out the voices of ordinary citizens."

Stevens is leaving 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.

Stevens' announcement, which came 11 days before his 90th birthday, had been hinted at for months.

It's presumed Obama will nominate another liberal, so the departure won't alter the court's philosophical makeup. Leading candidates to replace Stevens include Solicitor General Elena Kagan, 49, and federal appellate judges Merrick Garland, 57, in Washington and Diane Wood, 59, in Chicago.

Here, they are listed alphabetically, along with pros and cons of their possible nominations.

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Merrick B. Garland

Merrick B. Garland of the U.S. Circuit Court of Appeals for the District of Columbia Circuit. Garland was born in Chicago in 1952, and was nominated to the U.S. Court of Appeals for the District of Columbia Circuit in 1997 by President Bill Clinton.

A Harvard law graduate, he clerked for Supreme Court Justice William Brennan 1978-1979 before entering government service as a special assistant U.S. attorney general. Garland left the Justice Department for private practice in Washington, D.C. in 1981, where he stayed until 1993. He took a three-year break during that time to work as an assistant U.S. attorney for the District of Columbia. He was promoted to deputy assistant attorney general in the Justice Department's criminal division in 1993 and became principal associate deputy U.S. attorney general in 1994, where he stayed until his court nomination.

Pro: Well respected by conservative and liberal experts following his management of the investigation of the 1995 bombing of the Oklahoma City federal building and subsequent prosecution of bomber Timothy McVeigh. Moderate legal positions would not likely lead to filibuster threats from Republican senators.

Con: Moderate legal positions would not excite hard-core liberals with congressional elections coming up this year. Was not a finalist in the White House's last search for a Supreme Court nominee, which produced Justice Sonia Sotomayor. Obama may want to put another woman or minority on the court, instead.

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Gov. Jennifer Granholm

Michigan Gov. Jennifer Granholm. Granholm was born in 1959 in Vancouver, British Columbia. She got her law degree from Harvard University in 1987 and entered the political world by working as a full-time aide for the Michigan campaign for Democratic presidential candidate Michael Dukakis in 1988.

She got into the legal world in 1988 through her job as an executive assistant for criminal justice issues in the Wayne County executive office. Granholm started work as a prosecutor in the U.S. attorney's office in Detroit in 1990, where she stayed until her appointment as Wayne County, Mich., corporation counsel in 1995. She became Michigan's attorney general in 1999, and was became governor in January 2003.

Pro: Would bring nonjudicial experience to the Supreme Court, which several senators say is needed. Would bring the number of female Supreme Court justices to three, an all-time high. Would bring political and prosecutorial experience.

Con: No judicial experience. Has said nomination would be a great opportunity, but "I just don't think that's going to happen." Was not a finalist in the White House's last search for a nominee. Questions about her stewardship of Michigan, which was hit hard during the recession, could come up.

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Elena Kagan

Elena Kagan, U.S. solicitor general. Kagan was born in 1960 in New York City. She received a law degree from Harvard Law School in 1986 before working as a law clerk to Supreme Court Justice Thurgood Marshall. Kagan went into private practice in Washington, D.C. from 1989 until leaving to become a professor at University of Chicago law school in 1991.

She joined the Clinton administration as an associate counsel to President Bill Clinton in 1995 and climbed the ladder to deputy assistant to Clinton for domestic policy and deputy director of the Domestic Policy Council in 1997. Clinton nominated her to the U.S Court of Appeals for the District of Columbia Circuit in 1999 but she never received a hearing from the Senate Judiciary Committee. She became a professor at Harvard Law School since 1999, and became dean in 2003. She was confirmed as solicitor general in 2009.

Pro: Well respected by conservative and liberal lawyers after time at Harvard Law School. One conservative activist last year called Kagan "less extreme than most of President Obama's leading candidates for the Supreme Court." Already known as "Tenth Justice" because the solicitor general is the executive branch's chief lawyer before the high court. Was a finalist in the White House's last search for a nominee and was interviewed by the president. Would bring the number of female justices to three.

Con: No judicial experience. Thirty-one Republicans voted against her for solicitor general. Republicans refused to hold a hearing for her nomination to the U.S. Court of Appeals in 1999, a seat that went to eventual Chief Justice John Roberts. Got first Supreme Court argument experience in 2009 as solicitor general. Her stand against military recruitment at Harvard Law School because of the armed forces' "Don't Ask, Don't Tell" policy is sure to be a talking point against her. 

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Harold Hongju Koh

Harold Hongju Koh, legal advisor to the State Department. Koh, who has been the legal adviser to the State Department since March 2009, was born in Boston in 1954. He got his law degree from Harvard University before serving as law clerk for Justice Harry Blackmun in 1981-82. He went into private practice in Washington, D.C., for a year after his Supreme Court clerkship before being hired on at the Office of Legal Counsel at the Justice Department.

He left government to become a law professor at Yale University in 1985, where he stayed until he became assistant secretary of state for democracy, human rights and labor in 1998. He returned to Yale in 2001, and became Yale's law school dean in 2001. He left Yale to work for the State Department after Obama was elected.

Pro: Would make history as the first Asian-American Supreme Court nominee. Would excite hard-core liberals looking to swing the courts to the left. Would bring nonjudicial experience to the Supreme Court.

Con: No judicial experience. Liberal stances would guarantee a fierce confirmation fight with Republicans in the Senate, who have enough votes to filibuster a nominee if they want.

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Gov. Deval Patrick

Massachusetts Gov. Deval Patrick. Patrick was born in 1956 in Chicago, and got his law degree from Harvard University in 1982. He went to work as a lawyer for the NAACP Legal Defense Fund from 1983 to 1986 before entering private practice. He stayed in the private sector except for a stint in 1994-97 at the Justice Department. As the Clinton administration's assistant attorney general for civil rights he led a massive federal investigation of church burnings throughout the South.

Patrick left government to become chairman of Texaco Inc.'s Equality and Fairness Task Force, then was named vice president and general counsel for Texaco Inc. in 1999. He then became executive vice president and general counsel for The Coca-Cola Co. from 2001 to 2004 before winning the Massachusetts governorship as a Democrat in 2006.

Pro: Personal friend of Obama and like the president is a Chicagoan, Harvard Law School graduate and African-American. Would increase the number of black justices to two. Would bring political and prosecutorial experience to the high court.

Con: No judicial experience. Would face questions about the political and financial status of Massachusetts, which has a statewide health care plan similar to the nationwide plan just passed by Congress and signed into law by Obama.

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Diane Pamela Wood

Diane Pamela Wood of the 7th U.S. Circuit Court of Appeals in Chicago. Wood was born in 1950 in Plainfield, N.J., and was nominated to the 7th Circuit by President Bill Clinton in 1995. She got her law degree from the University of Texas School of Law before clerking for Supreme Court Justice Harry Blackmun from 1976-1977.

She went to work as a lawyer-adviser in the State Department's Office of the Legal Adviser