WASHINGTON -- Chief Justice William H. Rehnquist has thyroid cancer, a stunning disclosure Monday that caught even the closest Supreme Court observers off guard and injected into the presidential campaign the issue of appointments to America's most important legal panel.

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Rehnquist's diagnosis was announced in a terse statement issued by the Supreme Court. It said the 80-year-old widower who has led the court for a generation underwent a tracheotomy over the weekend and was hospitalized but expected to be back at work next week when the court resumes hearing cases.

Left unsaid was Rehnquist's condition at the National Naval Medical Center in suburban Bethesda, Md., and which type of thyroid cancer he has.

About 23,600 people develop various types of thyroid cancer each year in the United States. Most types are considered treatable, but many variables exist, including age and how quickly the cancer is found.

Dr. Yosef Krespi, chairman of otolaryngology at St. Luke's-Roosevelt Hospital in New York, said only aggressive or complicated thyroid cancers require a tracheotomy. Other physicians said the procedure is sometimes done as part of routine thyroid surgery.

Rehnquist's hospitalization gave new prominence to a campaign issue that has been overshadowed by the war on terror.

The next president probably will name one or more justices to a court that has been deeply divided in recent years on issues as varied as abortion and the 2000 election itself.

President Bush won that after the Supreme Court issued a key 5-4 decision in his favor, with Rehnquist as part of the majority.

The last court vacancy was in 1994, the longest stretch of continuity in modern history. Only one of the court's nine members -- Clarence Thomas, appointed by former President Bush -- is under 65.

"The Supreme Court has always been in play. This will just increase the salience," said Nelson Polsby, a political science professor at the University of California, Berkeley, and co-author of "Presidential Elections."

Rehnquist, a conservative named to the court in 1972 by President Nixon and elevated to chief justice by President Reagan in 1986, has had a series of health problems, including chronic back pain and a 2002 torn leg tendon that required surgery.

He was admitted to the hospital Friday and doctors performed the tracheotomy Saturday. During that procedure, a tube is inserted into the patient's throat, either to relieve a breathing obstruction or as preparation for surgery. The court did not explain why it was done on Rehnquist.

Edward Lazarus, a Los Angeles attorney and former Supreme Court clerk, said the secrecy was not surprising.

"The court doesn't appreciate being at the center of the political storm. It's an uncomfortable situation," he said.

Rehnquist is among the fiercest questioners during oral arguments. Dr. Herman Kattlove of the American Cancer Society said Rehnquist should be able to speak normally after the breathing tube is removed.

Three other members of the high court have had bouts with cancer. Justice John Paul Stevens, the oldest at 84, was treated for prostate cancer. Justice Sandra Day O'Connor had breast cancer and Justice Ruth Bader Ginsburg colon cancer.

Rehnquist turned 80 this month. The only older chief justice was Roger Taney, who presided over the high court in the mid-1800s until his death at 87.

White House press secretary Scott McClellan said President Bush and first lady Laura Bush "wish Chief Justice Rehnquist a speedy recovery." Democratic challenger John Kerry did not comment.

Rehnquist is a smoker known as a stern and efficient taskmaster at the court and a fierce competitor on the tennis court and at the poker table. In recent months his tennis is said to have been replaced with walks.

He has defied retirement rumors, even as some observers of the court have wondered aloud whether his conservative legacy -- empowering states, limiting abortion and preserving the death penalty -- might have run its course.

No matter who is elected president next week, a vacancy on the high court is likely during the next presidential term. And confirmation for any nominee is bound to be contentious due to the close split in the Senate.

Bush and Kerry have avoided describing a litmus test for a Supreme Court nomination, although their differences on abortion are cut along partisan lines. The future of Roe v. Wade, the three-decade-old ruling that affirmed the legality of abortion, is the most visible symbol of the court's ideological split.

Neither candidate has suggested names for possible nomination if a Supreme Court seat should become vacant during the next four years, but they have spoken about judges' approaches to specific issues.

About gay marriage, Bush said at the Republican convention: "I support the protection of marriage against activist judges, and I will continue to appoint federal judges who know the difference between personal opinion and the strict interpretation of the law."

Kerry has said he would nominate only Supreme Court justices who support abortion rights, and his campaign Web site says he would name "judges with a record of enforcing the nation's civil rights and anti-discrimination laws."

On Dec. 13, 2000, Rehnquist joined the four other more conservative justices in reversing Florida's court-ordered recount of presidential election ballots. The majority of the high court determined there was no time to conduct a lawful recount.

That decision resulted in Bush's being awarded Florida's 25 electoral votes, and the presidency, over Democrat Al Gore.

On Monday in Riviera Beach, Fla., Gore referred to the decision and said while he believes it was wrong he respects the court as an institution.

Rehnquist presided over then-President Clinton's 1998 impeachment trial in the Senate, which gave most Americans their first televised view of the chief justice.

Word of his illness comes as the Supreme Court deals with multiple legal fights stemming from the extremely tight election campaign.

On Saturday, the court refused to place independent presidential candidate Ralph Nader on the ballot in Pennsylvania. The high court has not yet acted on a similar appeal from Nader involving Ohio.

Concerns have arisen that the outcome of the presidential election could be challenged again, perhaps all the way to the high court.