Chief Justice William H. Rehnquist (search) revealed Monday that he is undergoing chemotherapy and radiation treatment for thyroid cancer, signs he has a grave form of the disease and probably will not return to the bench soon.

The election eve disclosure by the 80-year-old justice underscores the near certainty that the next president will make at least one appointment to the Supreme Court and probably more.

Rehnquist had planned to join his colleagues when they returned to hear arguments Monday after a two-week break. Instead, he issued a statement from home about the treatment he's receiving. It said he plans to work from home and made no mention of leaving the court.

The chief justice did not disclose what type of thyroid cancer he has, how far it has progressed or the prognosis.

Dr. Ann M. Gillenwater of the University of Texas M.D. Anderson Cancer Center (search) in Houston said the combination of chemotherapy and radiation is the usual treatment for anaplastic thyroid cancer, a fast-growing form that can kill quickly.

About 80 percent of people with that type of cancer die within a year, even with treatment, according to the Merck Manual of Diagnosis and Therapy (search).

"Unfortunately, it rarely responds very well, and this is just a holding action for most patients," said Dr. Herman Kattlove of the American Cancer Society (search).

Rehnquist's statement was a more somber announcement than the one a week ago, when he first made public that he had been hospitalized for cancer treatment but said he planned to be back at work in a week.

"According to my doctors, my plan to return to the office today was too optimistic," said Rehnquist, who spent a week in the hospital. "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."

News of Rehnquist's cancer has energized conservative and liberal groups, which have tried to draw voters' attention to the court's delicate balance on issues like abortion, affirmative action and the death penalty.

The spotlight would have been heightened in the final week of the campaign if Rehnquist had been more forthcoming about his condition, said Dennis Hutchinson, a Supreme Court expert at the University of Chicago Law School (search).

"He doesn't want to be a factor" in the election, Hutchinson said. "The one thing all members of the court hate is the assumption that they are partisan or sensitive to partisan politics."

Dave Rohde, a political science professor at Michigan State University, said Rehnquist's illness will probably not sway many last-minute undecided votes.

"This is not in the top tier of issues for voters. They're concerned about Iraq and terrorism and the economy and much less about this," he said.

Rehnquist, a Republican, has been the court's conservative leader for a generation. He voted with the other four conservative justices in the 5-4 Bush v. Gore ruling that decided the last presidential election. He has said he would be more likely to retire with a Republican in the White House.

Until now, Rehnquist's most serious health problem has been a bad back that often forces him to stand and stretch during arguments and has limited his tennis playing.

Rehnquist had been hoarse for several weeks before his hospitalization Oct. 22. He had a tracheotomy a day later, in which a tube was inserted in his throat to help him breathe.

Cancer of the thyroid, a gland in the neck that produces hormones to help regulate the body's use of energy, is diagnosed in about 23,600 Americans a year. The most common types are easily treatable.

Dr. Leonard Wartofsky, a thyroid cancer expert at Washington Hospital Center, said patients with anaplastic thyroid cancer often have four to six weeks of daily visits for radiation, and multiple rounds of chemotherapy that can span several months.

"It's exhausting for a young patient," Wartofsky said.

In his absence Monday, Justice John Paul Stevens, 84, presided over the court. The court's oldest member said Rehnquist could still vote in cases being argued this week, after reviewing transcripts and briefs.

Should Rehnquist be too sick to participate in cases, the other eight justices would act without him. Tie votes would uphold the lower court's decision.

No one has left the court since 1994, a modern record. Stevens and Justice Sandra Day O'Connor, 74, are considered those most likely to step aside after Rehnquist.

Should President Bush win re-election, possible high court nominees include J. Michael Luttig, who serves on the 4th U.S. Circuit Court of Appeals in Richmond, Va.; Samuel A. Alito, a judge on the 3rd U.S. Circuit Court of Appeals in Philadelphia; and Emilio Miller Garza, a Hispanic judge on the 5th U.S. Circuit Court of Appeals in New Orleans.

John Kerry administration possibilities include Merrick Garland and David Tatel, both appeals court judges in Washington, and two Hispanic judges: Jose A. Cabranes and Sonia Sotomayor, both of the 2nd U.S. Circuit Court of Appeals in New York.