The surprise resignation yesterday of Justice William J. Brennan Jr. confronts President Bush with a difficult decision and without an obvious choice to replace the leader of the court's liberal wing with someone of more conservative bent.

Immediate speculation centered on a number of judges named to appeals courts around the country by former president Ronald Reagan as well as on Kenneth W. Starr, who left the federal appeals court here to serve as the Bush administration's top lawyer before the Supreme Court.

In the Oct. 13, 1988, presidential debate, Bush was asked about what kind of people he would appoint to the Supreme Court. "I don't have any litmus test," he said. "But, what I would do is appoint people to the federal bench that will not legislate from the bench, who will interpret the Constitution. I do not want to see us go to again -- and I'm using this word advisedly -- a liberal majority that is going to legislate from the bench. . . . I won't support judges like that."

Bush said he thought Reagan's judicial selections had been "outstanding, outstanding appointments." In response to a question, he included failed Supreme Court nominee Robert H. Bork in that assessment.

The choice facing Bush is far less clear cut than was Reagan's selection -- ultimately unsuccessful -- of then-Appeals Court judge Bork to replace Justice Lewis F. Powell Jr. following Powell's 1987 resignation. Bork was strongly championed by conservatives who had seen him passed over twice for vacancies at the high court, and his selection -- while extremely controversial -- was not a surprise.

Bush, in contrast, faces no such obvious leading contender for Brennan's seat, finding himself in much the same position as Reagan following Bork's defeat. But, unless he chooses an extreme conservative or picks Bork again, Bush likely will not face the same firestorm of opposition that characterized the battle over Bork.

Sen. Paul Simon (D-Ill.), a member of the Senate Judiciary Committee, said on ABC-TV's "Nightline" last night that "if the president picks someone who is too far to the right ideologically, we'll have a big fight."

A senior White House official said last night that "serious looking" for a replacement already is underway, with Attorney General Dick Thornburgh, White House Counsel C. Boyden Gray and Chief of Staff John H. Sununu involved. "We didn't wait until two weeks ago to start thinking about this," the official said.

Bush said he planned to meet with those three officials this morning to discuss the vacancy.

The White House official said there is no list yet, but Starr, the current solicitor general, is "a star;" Trade Representative Carla A. Hills a "long-shot," and there are others.

The official said there was "no doubt in my mind" Bush will choose a nominee opposed to abortion rights because "it's not worth the pain" from conservatives already incensed about Bush's abandonment of his "no new taxes" pledge.

The announcement gives the administration a short time to act before the court starts its October 1990 term. There is no time limit for the president to announce a nominee or the Senate to vote, but Republicans are sure to press to act swiftly, before the term starts, while Democrats may argue that they need time to study the scholarly writings, decisions or other records of a nominee.

Congress starts its August recess early next month and will be out until the first week of September. In addition, in an election year, there will be pressure to finish up by early October.

Starr, at the top of the short list of possible nominees, turns 44 today. He is young, generally conservative and experienced, having spent six years as a judge on the D.C. Circuit before leaving to become solicitor general last year.

As a top aide to former Attorney General William French Smith, Starr argued against the administration's filing of a brief attacking the IRS policy of denying tax-exempt status to racially discriminatory private schools in the Bob Jones University case.

While on the D.C. Circuit, Starr overturned the constitutionality of the District's affirmative action program for firefighters, saying a "rigid quota based strictly on race" was an "impermissible" means of making up for past discrimination. He reversed a lower court ruling that the D.C. prison had violated the inmates' rights against cruel and unusual punishment.

He joined with the court's liberal members to reject a libel suit against The Washington Post by the former president of Mobil Corp. and said disparaging statements by columnist Rowland Evans and Robert Novak about a Marxist professor were "entitled to absolute First Amendment protection as expressions of opinion."

As solicitor general, Starr wrote a brief that once again told the court that the landmark Roe v. Wade abortion decision was wrongly decided and should be overruled. In this term's Nancy Cruzan case, involving the right to die, Starr joined with Missouri in arguing that the state should be able to demand a high standard of proof before allowing such drastic action. He unsuccessfully defended the constitutionality of the federal statute against flag burning.

Others mentioned as possible nominees include various appeals court judges, including Edith H. Jones, 41, and Patrick E. Higginbotham from the 5th Circuit; Ralph K. Winter Jr. of the 2nd Circuit; J. Harvie Wilkinson III of the 4th Circuit; Pamela A. Rymer of the 9th Circuit; and Clarence Thomas, 42, of the D.C. Circuit, as well as Hills.

Jones was a Houston lawyer and general counsel for the Texas Republican Party before being named to the 5th Circuit in 1985. In a letter last year to the New York Times, she criticized defense lawyers in death-penalty cases who "employ last-minute tactics as a cloak for a petition without merit."

Higginbotham, 51, was named by former president Gerald R. Ford to the federal trial court in Dallas in 1975 and was promoted to the 5th Circuit in 1982. Higginbotham could face opposition from anti-abortion groups concerned about his 1986 decision striking down Louisiana abortion laws, even though he noted that the court's "abortion jurisprudence has been subjected to exceptionally severe and sustained criticism."

Winter, 54, a former Yale Law School professor and clerk to Justice Thurgood Marshall, was frequently mentioned as a possible successor to Powell. He has been on the appeals court since 1982.

Wilkinson, 43, a former Powell clerk and top aide in the Justice Department's civil rights division, was named to the federal appeals court in Richmond in 1984. He received only a "qualified" rating from the American Bar Association then because he had never practiced law.

On the court, he voted to strike down Richmond's minority set-aside program for contractors and to give Hustler magazine publisher Larry Flynt constitutional protection from being sued over a parody of evangelist Jerry Falwell. Both views were later upheld by the Supreme Court.

Thomas, a former chairman of the Equal Employment Opportunity Commission and a black conservative, took Bork's seat on the D.C. Circuit earlier this year. His brief tenure on the court may be a factor that weighs against him.

Rymer, likewise, was only recently promoted to the 9th Circuit.

Hills, a former secretary of housing and urban development, was forced to defend her lobbying at HUD on behalf of a mortgage concern during hearings on the HUD scandal last year.

Staff writer David Hoffman contributed to this report.