Senate Majority Leader George J. Mitchell (D-Maine) yesterday said he turned down a chance to join the Supreme Court because he was concerned that the confirmation process could impede his efforts to steer health care and other administration initiatives through Congress this year.

While President Clinton made it clear he wanted to nominate him, Mitchell said, the president "reluctantly agreed" with Mitchell's assessment about the time-consuming nature of confirmation proceedings and other complications "which neither he nor I could now foresee."

Mitchell's announcement shocked Senate colleagues still reeling from his decision only a month ago to retire from the Senate. At the White House, officials who had been preparing for a quick announcement of a Mitchell selection were taken by surprise and cautioned that it could be several weeks before Clinton settles on a successor to Justice Harry A. Blackmun, who announced his resignation last week.

Mitchell told reporters he met with Clinton late Monday to tell him he could "best serve by concentrating this year on health reform and the rest of the president's agenda." Clinton asked Mitchell "to sleep on it," White House press secretary Dee Dee Myers said.

When Clinton telephoned Mitchell yesterday morning, Mitchell said his assessment remained the same, Myers said. "The president regretfully accepted Senator Mitchell's judgment on that" and did not try to persuade him to change his mind, she said.

Speaking at the White House yesterday, Clinton said Mitchell had told him, "I'm sorry that the timing is not good, but I think it's the right thing to do." He said the 60-year-old majority leader "seemed as comfortable with that decision as any one I've ever seen him make" and praised Mitchell "for his willingness to forgo a great personal opportunity."

One White House official said that Mitchell was "the first among equals." Said another: "I don't see anything happening in the near term. I don't think there's an easy decision in front of them."

Among those who now move to the forefront of consideration are federal judge Jose Cabranes of Connecticut, who would be the court's first Hispanic justice; federal appeals court judges Amalya L. Kearse of New York, who would be the court's first black woman and Richard S. Arnold of Arkansas, a longtime friend of the president's; and Solicitor General Drew S. Days III.

White House officials said Mitchell's withdrawal made it more likely the president would turn to a minority candidate, such as Cabranes, Kearse or Days, in part because there is a prospect that he will have another vacancy next year and may want to keep open the option of selecting Mitchell then.

The officials said Clinton would like to name Arnold, a highly respected jurist, but thought that was unlikely in the current atmosphere of charges that the president has relied too heavily on Arkansas friends to staff his administration. Arnold has also had health problems that could cause a concern for the administration.

Another federal appeals court judge, Stephen G. Breyer of Massachusetts, who was interviewed for the Supreme Court opening last year but lost out to Justice Ruth Bader Ginsburg, remains under consideration, officials said.

"I think there are a few leading candidates, not a leading candidate," one senior official said. But this official and others said the list could expand and said they did not expect Clinton to get to the stage of interviewing candidates for a few weeks.

Mitchell's announcement continues the White House losing streak of finding takers for a job most lawyers view as the height of their profession. Last year, after the retirement of Justice Byron R. White, New York Gov. Mario M. Cuomo (D) amazed the White House by rejecting Clinton's offer to name him to the vacancy. New York Court of Appeals Judge Judith Kaye took herself out of the running, and Education Secretary Richard Riley said he did not want to be considered.

Mitchell said yesterday that during his hour-long meeting, Clinton "told me that he wanted to appoint me to the court, that he intended to appoint me to the court, but that he was concerned, as I was, about the prospect {that} the nomination would affect my ability to serve as majority leader for the remainder of this session."

Mitchell said he thought he could have continued as majority leader while going through the confirmation process but questioned whether he could have done it well enough to suit his own standards.

"I think it's going to require every bit of energy and effort and concentration that I have to pass {health care} and the other important measures.... ." he said.

With an advance blessing from Minority Leader Robert J. Dole (Kan.) and many other Republicans, Mitchell had been expected to breeze through confirmation hearings, and many of his colleagues expressed dismay at his decision.

"It's a shock, I'll tell you, a shock," said Sen. Tom Harkin (D-Iowa). "Here's someone who could have had it in a walk."

"I'm disappointed for him but glad for us," said Sen. Daniel Patrick Moynihan (D-N.Y.).

But others suggested that the decision was in line with Mitchell's longtime dedication to health care reform and his punctilious observance of proper conduct.

"George is a perfectionist ... he always wants to do it right, " said Sen. Edward M. Kennedy (D-Mass.).

"Considering his concern about health care, I'm not all that surprised," said Sen. Thomas A. Daschle (D-S.D.), a candidate to succeed Mitchell as majority leader next year.

Some senators said they believed that Mitchell might have been more tempted to try to juggle his last few months as majority leader with the pressures of the confirmation process if the job were chief justice rather than associate justice.

Asked if he would take that position, Mitchell said he has no reason to believe that Chief Justice William H. Rehnquist is stepping down. If the post were offered, he would consider it, Mitchell said.

He said his decision was not influenced by questions involving a constitutional provision that bars a member of Congress from being named to a position for which pay has been raised during the member's term.

Nor, he said, did his decision signal that he has decided to become commissioner of major league baseball. As he has before, he said he has not been offered the job but would consider it if an offer was made.