Washington ? Chief Justice William Rehnquist was unable to preside over the Supreme Court Monday as planned, and specialists said his treatment indicated he had a particularly virulent, usually fatal form of thyroid cancer.

Rehnquist’s illness raises the specter of a possible tie vote on the court if today’s presidential election ends up in the courts, as many fear it could. In the case of a tie, the decision of the lower court is affirmed.

But Ted Olson, the lawyer who represented President Bush before the Supreme Court after the last election, said Rehnquist could simply resign and Bush could appoint his replacement as a highly unusual recess appointment without any Senate debate. Bush would in effect be appointing the person who might decide his own election, and such an appointment would create a political firestorm.

Rehnquist revealed in a statement issued Monday that he is being treated with radiation and chemotherapy, which doctors said was used in thyroid cancer only for the anaplastic form of the disease. While most forms of thyroid cancer are curable, anaplastic is almost always fatal, they said.

“The form of treatment he’s getting almost certainly indicates he is dealing with the most aggressive form of thyroid cancer, i.e., anaplastic,” said Dr. Irwin Klein, chief of endocrinology and metabolism at North Shore University Hospital in Manhasset, N.Y.

Another specialist said the disease was “unstoppable” and that treatment was merely an attempt to prolong Rehnquist’s life a few months.

“According to my doctors, my plan to return to the office today was too optimistic,” said Rehnquist, who is 80. “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.”

Without Rehnquist’s vote four years ago, the court would have tied on the procedural question of allowing the Florida recount to continue, giving Democratic candidate Al Gore at least a temporary victory. The makeup of the court has not changed since.

If a justice becomes disabled but refuses to resign, there have been times when the other justices have agreed to nullify his vote by agreeing to rehear cases where the ill judge would break a tie.

But Olson, who recently returned to private practice after four years as solicitor general, told Newsday that Rehnquist, a highly partisan Republican before he was appointed to the bench by Richard Nixon in 1971, could resign and be replaced immediately by anyone the president chooses.