The court said the chief justice would leave Bethesda Naval Hospital this week and would be on the bench next Monday in time for the next round of arguments.
Nevertheless, the news of the chief justice's illness, announced in late morning, created an immediate stir in political circles, with some saying it could raise voters' awareness of the prospect that the next president would have Supreme Court vacancies to fill.
Republicans tended to play down the potential impact, while Democrats saw it as an opportunity to try to win over ''persuadable'' voters who had not yet committed to a candidate.
''Anybody who is sophisticated enough to worry about the makeup of the Supreme Court has already decided whom they're going to vote for,'' said Neil Newhouse, a Republican pollster. ''So it may be important, but it's important to people who've already decided how they're going to vote.''
Similarly, Connie Mackay, a lobbyist for the Family Research Council, which promotes the conservative side of social issues, said of the news, ''I don't think it changes anything because the court has not been as out front as an issue as it has been in past elections.'' But Mackay said her organization would send e-mails ''reminding our constituents that once again, we've got to think clearly and get out to the voting polls so that the right person is elected to make the right appointments.''
Liberal groups said they hoped to use the Rehnquist news to prod uncommitted voters to choose Sen. John Kerry.
Elizabeth Cavendish, interim president of NARAL Pro-Choice America, which supports abortion rights, said that ''the fragility of the court has always been an issue.''
But, she said, the Rehnquist news ''crystallizes the point that many of the justices are older or have health problems and the next president will either protect or overturn a woman's right to choose.''
She said her group's final campaign literature was already in the mail but that it would probably send last-minute e-mails to voters whom it has identified as favoring abortion rights but who are either ''soft'' in their support for Kerry or planning to vote for Bush.
''The way to make a difference in an election is to reach new people who have some sort of gap in their thinking,'' Cavendish said.
Ralph Neas, president of People for the American Way, a liberal group, said that ''there is no question that the timing of this allows us to raise this issue and put it in play.'' He noted that a recent Time magazine poll reported that 86 percent of people surveyed said that appointments to the Supreme Court over the next four years are either very important (59 percent) or somewhat important (27 percent) to their voting decision.
When asked in the poll, on Oct. 15, whether the issue of Supreme Court appointments would make them more likely to vote for Bush or Kerry, 38 percent said it would make them more like to vote for the president and 43 percent said the issue would make them more likely to vote for his Democratic challenger.
Rehnquist, who turned 80 on Oct. 1, is the second-oldest person ever to serve as chief justice. The oldest was Roger Brooke Taney, who died in office in 1864 at the age of 87.
Thyroid cancer in general is readily treatable, with a cure rate of up to 90 percent. But a small minority of patients have a distinct type of thyroid cancer, anaplastic, that is aggressive and often rapidly fatal. The court provided no information on the chief justice's type of cancer or on his treatment for it, so it was not possible to assess his specific prognosis. Thyroid cancer is treated by surgery or, less often, by radiation. The court declined to say whether the chief justice had surgery in addition to the tracheotomy, which involves placing a tube through a hole in the throat to prevent or alleviate breathing problems.
