WASHINGTON — President Obama said yesterday that he plans to move quickly to name a replacement for retiring Supreme Court Justice David H. Souter, as senators and interest groups on both ends of the ideological spectrum began mobilizing for the first court confirmation battle of his presidency.

Making a surprise appearance in the White House briefing room after speaking with Souter by phone, Obama told reporters he hopes to have a new justice in place for the start of the court's next term in October, a timeline that aides said would require a nomination to be sent to the Senate by June or July.

The president praised Souter, 69, as a "fair-minded and independent judge" and said he would nominate a replacement who both respects the Constitution and brings "empathy" and "understanding" to the bench.

"I will seek someone who understands that justice isn't about some abstract legal theory or footnote in a casebook, it is also about how our laws affect the daily realities of people's lives — whether they can make a living and care for their families, whether they feel safe in their homes and welcome in their own nation," Obama said.

The vacancy presents Obama with his first opportunity to begin reshaping the court. With the Republican opposition in the Senate weakened by the November elections and this week's defection by Pennsylvania Sen. Arlen Specter, few conservatives held out much hope that they could block an Obama nominee.

On Capitol Hill, Republicans acknowledged the long odds they faced to defeat the eventual nominee unless Obama makes a selection that is easily portrayed as outside the mainstream of legal thinking.

"The only way the Obama administration can screw this up is to nominate someone who is a radical," said Sen. Lindsey O. Graham, R-S.C., a senior member of the Judiciary Committee. Graham said Republicans understood that "you're basically going to replace one liberal with another."

The only chance for a real surprise in the hearings, Graham said, would be if the White House slips up as it has with several Cabinet nominees and puts forward a candidate with financial or ethical problems.

"I would advise they vet the person a little better than they have in the past," Graham quipped.

POSSIBLE CHOICES

Early speculation about Obama's potential choices has centered on several female jurists, as well as a number of prominent Hispanics. A Hispanic has never served on the Supreme Court.

White House officials began the work of identifying potential court nominees soon after Obama's election in November. During the transition, aides began compiling a list of possible Supreme Court candidates, with Obama adding several names to the mix. That process accelerated when the White House began receiving suggestions in February that Souter hoped to step down.

Souter had not planned to announce his retirement until the end of the court's term in June, according to an informed source, and was stunned when the first reports broke Thursday night. The source said Souter called a friend Thursday evening to say that there was a story moving about his retirement and lament that he had not yet had a chance to convey his decision to Obama.

Souter called Obama yesterday and formalized his decision to retire with a one-paragraph letter to the president that was released by the court. Souter, known as the court's most reclusive member, made no public statement.

The pending retirement stoked immediate interest from both liberal and conservative activists. Both sides have begun mobilizing supporters in anticipation of a pitched confirmation battle, even though the appointment of a new justice is unlikely to alter the balance of a court that is split fairly evenly between liberals and conservatives.

FINDING SUPPORT

Conservative activists held conference calls, worked to raise the millions of dollars they would need for a public relations campaign targeting a nominee, and sought to activate networks of supporters to oppose Obama's choice, saying the criteria laid out by the president go beyond what is necessary for choosing a justice.

"He says he wants to appoint judges who show empathy, but what does that mean?" said Wendy Long, chief counsel to the Judicial Confirmation Network. "Who do you have empathy for? If you have empathy for everybody, you have empathy for nobody."

With the Senate on the verge of a 60-vote filibuster-proof Democratic majority, conservative groups said they see little chance of derailing a nominee they find objectionable. But armed with polling they say shows the vast majority of Americans favor judges who "interpret the law as written" without regard to their view and experiences, they see the Supreme Court nomination as a potential rallying point.

Liberal advocacy groups urged Obama to nominate a justice who will defend the powerless.

"President Obama has said that the Supreme Court must play a role in protecting people who are 'vulnerable in the political process: the outsider, the minority, those who are vulnerable, those who don't have a lot of clout,' " said Michael B. Keegan, president of People for the American Way. "We agree, and urge the president to nominate a justice who will defend the rights of individual Americans against powerful government and business interests."

Officials said there are fewer than 10 names on a list of would-be nominees, which will be winnowed as candidates are further vetted in the coming weeks.

Most often mentioned as possibilities are two appeals court judges, Sonia Sotomayor of New York and Diane P. Wood of Chicago, along with Obama's new solicitor general, Elena Kagan. Other names that have been mentioned include Massachusetts Gov. Deval L. Patrick and Georgia Supreme Court Chief Justice Leah Ward Sears.