ASHINGTON, Nov. 1 - Chief Justice William H. Rehnquist disclosed Monday that his thyroid cancer was being treated with both chemotherapy and radiation, and he did not return to work despite his previously announced plan to do so.

A carefully worded statement released by his office shortly before the other eight justices began hearing arguments gave no indication when, or whether, the 80-year-old chief justice might return to the bench.

That silence invited immediate speculation that he would soon retire. The doctors who are treating him have not made any public statements. But medical specialists not connected with his case said his course of treatment strongly suggested that he could be suffering from a rapidly progressive type of cancer that had already spread and might now be inoperable. This would make it unlikely that he could complete the court's current term.

The statement was an extraordinary development in the closing hours of a presidential campaign in which partisans on both sides have tried to remind voters that the next president could well be in a position to reshape the Supreme Court.

The court has gone without a vacancy for more than 10 years and is closely divided on many important issues. A bitter Supreme Court confirmation fight could serve to reinforce the political polarization that has characterized the presidential race.

Chief Justice Rehnquist spent seven days at Bethesda Naval Hospital, returning home on Friday. While in the hospital, he underwent a tracheotomy to relieve breathing problems, but the clear implication of the statement was that the cancerous thyroid itself had not been removed.

Removal of the thyroid is the usual treatment for the more common, readily curable form of thyroid cancer. While the chief justice gave no details on his type of cancer, doctors who are not involved in his treatment said the information that he did provide suggested that he might have the anaplastic type, which is typically fatal in a relatively short time.

In his statement, Chief Justice Rehnquist said he was recuperating at home and would have radiation and chemotherapy treatment on an outpatient basis. "According to my doctors, my plan to return to the office today was too optimistic," he said.

The Supreme Court's press office announced that plan last Monday, when it said that the chief justice "underwent a tracheotomy on Saturday in connection with a recent diagnosis of thyroid cancer."

While the court provided no other details at that time, cancer specialists said then that the tracheotomy indicated serious illness and made his speedy return to work unlikely.

The latest statement, issued not by the press office but from the chief justice's chambers, concluded: "While at home, I am working on court matters, including opinions for cases already argued. I am, and will continue to be, in close contact with my colleagues, my law clerks, and members of the Supreme Court staff."

Justice John Paul Stevens, the senior associate justice, who presided over the court's proceedings on Monday, noted that the chief justice "reserved the right" to participate in cases by reading the briefs and the transcripts of the arguments.

Justices occasionally vote in cases in which they have missed the argument, but typically do not. A 4-to-4 tied vote serves under the court's rules to affirm the lower court opinion without setting a precedent for other cases.

Political analysts said Monday that the latest news about the chief justice would probably have little effect on the election because voters who care about the composition of the court had already made up their minds.

The political ramifications of filling a vacancy on the court would depend in part on when it occurred. The picture could be tremendously complicated if a vacancy were to occur while it appeared that a contested election was heading once again for the Supreme Court.

Todd F. Gaziano, director of the Center for Legal and Judicial Studies at the Heritage Foundation, a conservative research group, said that if President Bush lost the election on Tuesday but the result was contested in the courts, Mr. Bush would "have the responsibility to consider" making a recess appointment to fill a Supreme Court vacancy.

Such an appointee could serve without Senate approval until the end of the first session of the new Congress in late 2005.

After the 2000 election, President Bill Clinton used his constitutional authority to make recess appointments to place Roger L. Gregory on the federal appeals court in Richmond, Va. The Senate later confirmed Judge Gregory to a lifetime position. President Eisenhower used recess appointments to place three justices on the Supreme Court: Earl Warren as chief justice and Potter Stewart and William J. Brennan Jr. as associate justices. The Senate then confirmed all three.

However, the political climate now is very different, and Democrats said Monday that a recess appointment by a lame-duck President Bush would be politically unthinkable. John Podesta, chief of staff in the Clinton administration and now president of the Center for American Progress, a liberal research group, dismissed the idea as a preposterously "in your face" maneuver.

Any Supreme Court vacancy that occurred soon could become the vehicle for the bitter campaign of 2004 to perpetuate itself, even if the election outcome is clear. Mr. Podesta said that if the Senate remained as closely divided as it is now, "neither party will be in a strong position to push through a nominee."

He predicted that given the divisiveness of the presidential campaign, Senator John Kerry, if elected, would use a vacancy to send a signal that he intended "to bring the country together back toward the center" while Mr. Bush would try to use a vacancy as "pay back" to satisfy his conservative supporters.

Tony Perkins, president of the conservative Family Research Council, said that a President Kerry would "have some difficulty" with his nominees if the Senate remained Republican. Kerry nominees "won't be pro-life and they won't be pro-traditional marriage," Mr. Perkins said, adding that "you will see a strong battle waged on both sides" because "people realize what is at stake and neither side will relent in their efforts to see that their issues are guarded."

The leaders of two liberal groups, Nan Aron of the Alliance for Justice and Ralph Neas of People for the American Way, both said that animosity between Mr. Bush and the Democrats predated the presidential campaign because of fights over lower-court nominations and agreed that the election would only have worsened the situation. Ms. Aron added that those fights had only primed Senate Democrats for battle.

Any president faced with a vacancy for chief justice must decide whether to choose someone from outside the court or to elevate an associate justice, which gives him the chance to pick two nominees but also requires two confirmation efforts.

William Rehnquist faced a prolonged debate when President Ronald Reagan nominated him in 1986 for elevation to chief justice to replace Warren E. Burger. He was confirmed by a vote of 65 to 33, not much different from the 68-to-26 vote by which he was confirmed as President Richard M. Nixon's nominee to the court in 1971.