Prime Minister Sharon in 'Induced Coma'

Olmert, 60, is a staunch supporter of Sharon who joined him in leaving the Likud Party for Kadima. Considered a blunt-spoken pragmatist, Olmert has held several cabinet posts; he currently serves as finance minister as well as vice premier. He served two terms as Jerusalem's mayor and was a strong supporter of Israel's withdrawal from the Gaza Strip last year, a move conceived of and dictated by Sharon.

Although Sharon had been quiet about his plans should he be reelected in March, he has committed himself publicly to the U.S.-backed peace plan known as the "road map," which calls for the creation of a Palestinian state alongside Israel.

Word of Sharon's condition spread quickly Wednesday, and there was swift reaction.

"I fear for his life," Yosef Lapid, the leader of the centrist Shinui Party, told Israel's Channel 2. "This is one of the most dramatic nights in the history of the state of Israel."

The spiritual leader of the ultra-Orthodox party Shas, Ovadia Yosef, called for prayer. So did Israel's two chief rabbis, who have been at odds with some of the prime minister's policies over the past year.

"In these moments, it becomes clear that what should stand at the top of our priorities is the concern for Prime Minister Ariel Sharon's health," said Eli Yishai, a Shas member of the Knesset, Israel's parliament.

Sharon's military and political careers run through some of the most pivotal moments in the country's 58-year history. Since becoming prime minister in 2001, he has charted a political course that bewildered enemies and allies as he redrew the country's ambiguous border with the Palestinian territories.

Last year, Sharon turned against Likud, the hawkish party he helped found, when he engineered the withdrawal of 8,500 Israeli settlers from the Gaza Strip. The enormous military operation ended Israel's 38-year presence in Gaza, a collection of 21 settlements and military bases that Sharon once championed as a strategic buffer against a potential Egyptian invasion.

The Gaza withdrawal served notice to Sharon's former political base that he intended to redefine Israel's borders in order to ensure its security and the long-term viability of its Jewish majority. Once a proponent of defining the boundaries of the Jewish state as including Gaza and the West Bank, Sharon has rolled back those territorial ambitions in large part because the fast-growing Palestinian population in the territories threatens to overtake the Jewish majority.

In withdrawing from Gaza, Sharon declared his intention to hold on to Israel's major settlement blocs in the West Bank under any final peace agreement. To do so, he ordered the construction of a 400-mile-long barrier that cuts into land envisioned as part of a future Palestinian state.

Sharon is vilified in much of the Arab world for these policies, as well as for his role as the architect of Israel's 1982 invasion of Lebanon. He was removed as Israel's defense minister a year later after an Israeli investigation into the massacre of hundreds of Palestinians in the Sabra and Shatila refugee camps south of Beirut found that he bore "indirect responsibility" for the incident, carried out by Christian militias allied with Israel in Lebanon's civil war.

The medical emergency Wednesday developed just hours before Sharon was scheduled to undergo a procedure to repair a small hole between the upper chambers of his heart, a birth defect doctors discovered after he suffered a minor stroke last month. Doctors said the hole likely played a role in causing the Dec. 18 stroke, which led them to give Sharon blood-thinning medication.

Soon after Sharon complained of chest pain on Wednesday evening, his physician rushed to Sycamore Ranch, his farm in the Negev. After an examination, Sharon was taken by ambulance on the roughly hour-long ride to the hospital. Ron Krumer, a hospital spokesman, said Sharon arrived at 10:56 p.m. local time and underwent tests in the trauma ward.

Spokesmen from the prime minister's office said he did not lose consciousness during the trip to the hospital, where he was scheduled to undergo a heart catheterization Thursday morning. His doctors have said in recent interviews that his age and excessive weight made the relatively simple procedure riskier.

Sharon's December stroke was apparently caused by a blood clot that passed through the small hole in his heart and traveled to his brain. Sharon's physicians did not reveal where that clot had originated, and tests might not have detected it. They did say, however, that the blood-thinning medicine the prime minister was taking had been prescribed to prevent more clots from forming.

A stroke is the death of brain tissue caused either by the blockage of a blood vessel -- in which brain cells are deprived of oxygen and nutrients -- or, as in this case, the rupture of a vessel -- leading to a cerebral hemorrhage.

Cerebral hemorrhage, or bleeding in the brain, is a rare and potentially lethal complication of blood thinners such as heparin and warfarin. In severe cases, blood pours out of a ruptured vessel, raising pressure inside the skull and compressing brain tissue. That, in turn, can damage or kill neurons, and in some cases can even squeeze the base of the brain through the hole that connects it to the spinal cord, causing death.

Surgeons can deal with large hemorrhages by drilling a hole through the skull and removing the clot with a suction device. That procedure, however, also can be dangerous. It is often difficult to quickly restore blood coagulation to a normal level, and even in that case, the patient risks facing the original problem.

Staff writer David Brown in Washington contributed to this report.