President Barack Obama’s first pick for the high court is likely to be a liberal-leaning nominee, much like Souter.

The White House has been told that Souter will retire in June, when the court finishes its work for the summer, a source familiar with his plans said Thursday night. The retirement is likely to take effect only once a successor is confirmed.

The source spoke to The Associated Press on condition of anonymity because he was not authorized to speak for Souter.

Souter had no comment Thursday night, a Supreme Court spokeswoman said.

The vacancy could lead to another woman on the bench to join Justice Ruth Bader Ginsburg, currently the court’s only female justice.

At 69, Souter is much younger than either Ginsburg, 76, or Justice John Paul Stevens, 89, the other two liberal justices whose names have been mentioned as possible retirees. Yet those justices have given no indication they intend to retire soon and Ginsburg said she plans to serve into her 80s, despite her recent surgery for pancreatic cancer.

Souter, a regular jogger, is thought to be in excellent health.

Interest groups immediately began gearing up.

“We’re looking for President Obama to choose an eminently qualified candidate who is committed to the core constitutional values, who is committed to justice for all and not just a few,” said Nan Aron, president of the liberal Alliance for Justice.

Some of the names that have been circulating include recently confirmed Solicitor General Elena Kagan; U.S. Appeals Court Judges Sonya Sotomayor, Kim McLane Wardlaw, Sandra Lea Lynch and Diane Pamela Wood; and Leah Ward Sears, chief justice of the Georgia Supreme Court. Men who have been mentioned as potential nominees include Massachusetts Gov. Deval Patrick, Harvard Law professor Cass Sunstein and U.S. District Judge Ruben Castillo of Chicago.

The Obama White House began from almost its first days in office preparing for the possibility of a retirement by thinking about and vetting potential high court nominees. Those efforts only accelerated with Ginsburg’s cancer surgery.

The timing may have been unexpected, but Souter has long yearned for a life outside Washington.

He has never made any secret of his dislike for the capital, once telling acquaintances he had “the world’s best job in the world’s worst city.” When the court finishes its work for the summer, he quickly departs for his beloved New Hampshire.

He has been on the court since 1990, when he was an obscure federal appeals court judge until President George H.W. Bush tapped him for the Supreme Court.

Bush White House aide John Sununu, the former conservative governor of New Hampshire, hailed his choice as a “home run.” And early in his time in Washington, Souter was called a moderate conservative.

But he soon joined in a ruling reaffirming woman’s right to an abortion, a decision from 1992 that remains still perhaps his most noted work on the court.

Souter became a reliable liberal vote on the court and was one of the four dissenters in the 2000 decision in Bush v. Gore that sealed the presidential election for George W. Bush.

Yet as Souter biographer Tinsley Yarbrough noted, “he doesn’t take extreme positions.” Indeed, in June, Souter sided with Exxon Mobil Corp. and broke with his liberal colleagues in slashing the punitive damages the company owed Alaskan victims of the Exxon Valdez oil spill.

Souter is the court’s 105th justice, only its sixth bachelor. He works seven days a week through most of the court’s October-to-July terms, a pace that he says leaves time for little else. He told an audience this year that he undergoes “an annual intellectual lobotomy” each fall.

Souter earned his bachelor’s and law degrees from Harvard sandwiched around a stay at Oxford University as a Rhodes scholar.

He became New Hampshire’s attorney general in 1976 and a state court judge two years later. By 1990, he was on the federal appeals court in Boston for only a few months when Bush picked him to replace Justice William Brennan on the Supreme Court.

But his unlikely legacy as gay rights pioneer was sealed May 15, when he heard the roar of a crowd gathered below his office as his majority decision legalizing same-sex marriage was announced.

Now, the law-and-order supporter of capital punishment is enduring from gay marriage foes the very complaints of “judicial activism” he has worked so hard to avoid during his 17 years on the high court and 34 years as a California judge.

He will likely have to mount an aggressive and expensive campaign to retain his seat in the 2010 election.

“Absolutely, Ron George should be thrown out for voting for gay marriage,” said Mike Spence, president of the conservative California Republican Assembly. “He has a very radical view of what’s a family.”

George makes no apologies for taking the lead on a politically dangerous case.

“I really felt that as chief justice I had to have the broad shoulders because I knew there would be substantial controversy about it,” he said in a recent interview.

The landmark decision overturned California’s bans on same-sex marriage, extending to sexual orientation the same civil rights protections afforded to race, religion and gender. Decisions by the California Supreme Court are often followed by state courts elsewhere.

Opponents have gathered signatures to put a measure on the November ballot for a constitutional amendment that would again ban gay marriage. George declined to discuss the court decision in detail, citing the measure and the legal challenges expected regardless of the election’s outcome.

Until he wrote the 4-3 majority decision, George was more noted for his administrative achievements and political prowess than his court decisions.

Four governors named him five times to higher judicial positions, starting with Ronald Reagan, who appointed him to the Los Angeles County Municipal Court in 1972.

Since 1996, when he was appointed the state’s top judge by then-Gov. Pete Wilson, George has worked tirelessly with the Legislature to modernize the state’s sprawling court system.

He shifted funding of the system from individual counties to the state, ensuring consistent budgets. He also combined the Byzantine municipal and superior court systems into one unified branch of government.

The marriage opinion surprised the legal community, which widely expected the court to uphold California’s gay marriage ban, said Santa Clara University law professor Gerald Uelmen.

“Most people thought his legacy would be the modernization of the courts,” Uelmen said. “I think the gay marriage decision will now be his principal legacy.”

George began his legal career as a state prosecutor after graduating from Stanford University’s law school in 1964. He appeared in 1972 before the U.S. Supreme Court in a futile bid to defend California’s death penalty law; capital punishment was legalized five years later.

On the bench, George was noted as a tough-on-crime jurist. As a trial judge in Los Angeles, he refused to let prosecutors dismiss murder charges against “Hillside Strangler” Angelo Buono, who was ultimately convicted on nine counts of murder.

Yet George left hints of what some would call more liberal leanings.

In 1995, as an associate justice, he wrote the high court’s majority opinion holding that private country clubs that conduct business with the public must allow women to join.

In 1997, he wrote the decision allowing girls under 18 to undergo abortions without their parents’ permission.

The next year, George raised $700,000 to defeat a campaign to unseat him because of that ruling. His opponents raised only $40,000, and the chief justice easily retained his seat with 75 percent of the vote.

California’s seven Supreme Court justices must be retained by voters every 12 years.

Political conservatives vow to organize a campaign to oust him because of the gay marriage decision, though they haven’t formally started raising money.

Spence said his group and others will formally organize after the November election.

The impeccably mannered George, who rarely displays displeasure in public even when sitting through the weakest of legal arguments in his courtroom, becomes slightly annoyed with the subject of mounting a political campaign.

“I have no idea,” he says when asked when he will formally launch his campaign. “It isn’t anything that I have given any thought to. I will do what I should do when that time comes about.”