The spiritual leader of the ultra-Orthodox Shas party, which holds 11 seats in the 120-member Israeli parliament, was seriously ill in hospital on Monday, one of his doctors said.
Rabbi Ovadia Yosef, 93, was on a respirator after undergoing heart surgery in Jerusalem's Hadassah hospital, cardiologist Dan Gilon told Israeli television.
"His condition is serious but not irreversible," Gilon said.
Yosef, whose son Yitzhak was elected chief rabbi of Israel's Sephardic Jews in June, a post he himself had previously held, has been in and out of hospital for several months.
He wields enormous influence among Israeli Jews of Middle Eastern and North African ancestry, and has frequently been a kingmaker in the country's fickle coalition politics.
Shas was a member of successive governing coalitions before going into opposition after the last general election in January.
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The party strongly opposed the demands of the secular rightwing coalition partners of Prime Minister Benjamin Netanyahu for more sweeping conscription of the ultra-Orthodox, who have long enjoyed extensive exemptions from military service.
Yosef founded Shas in 1984 on the platform of a return to religion and as a counter to an establishment dominated by Ashkenazi Jews of European ancestry.
The Baghdad-born rabbi has frequently sparked controversy.
In the past, he has referred to Palestinians and other Arabs as "snakes" and "vipers" who were "swarming like ants."
He called on God to strike down then prime minister Ariel Sharon over Israel's 2005 withdrawal from Gaza, and during the 2006 war in Lebanon implied that Israeli soldiers killed in battle died because they didn't follow Jewish commandments.
He also sparked outrage in 2000 when he said the six million Jewish victims of the Nazi Holocaust did "not die for nothing," but were the "reincarnation of Jews who had sinned" in previous generations.