Blinken urges Mideast leaders to press Hamas over a cease-fire.

But Mr. Gantz’s move means that Mr. Netanyahu is now totally dependent on his far-right and ultra-Orthodox coalition partners as he prosecutes the war in Gaza in the face of mounting international opprobrium, leaving him increasingly isolated and exposed at home and abroad.

Mr. Gantz and Gadi Eisenkot, another powerful member of National Unity, also left Mr. Netanyahu’s small war cabinet. They are both former military chiefs who were widely viewed as key voices of moderation in the five-member body, which was formed in October after the Hamas-led assault on Israel prompted the Israeli bombardment and ground invasion of Gaza.

The two centrist politicians raised public confidence in the government’s decision-making at a time of national trauma. They also lent the war cabinet an aura of legitimacy and consensus as Israel fought Hamas in Gaza, as well as its archenemy Iran and its other proxies, including the powerful Hezbollah militia across Israel’s northern border with Lebanon.

Mr. Gantz accused Mr. Netanyahu of “political procrastination,” suggesting that he had been putting off critical strategic decisions to ensure his political survival. His decision to quit the wartime government ushers in a new period of political instability and has left many Israelis wondering where the country goes from here.

Describing the political shake-up as “incredibly consequential,” Yohanan Plesner, president of the Israel Democracy Institute, a nonpartisan research group in Jerusalem, said that Israelis had already been giving low grades to the government on a host of wartime issues. That included the handling of the fighting and relations with the United States, Israel’s crucial ally, he said.

“With Gantz’s absence, I expect those grades to become even lower,” Mr. Plesner said.

Mr. Gantz had issued an ultimatum three weeks ago, warning Mr. Netanyahu that he would break up the emergency government unless the prime minister came up with clear plans, including on who would replace Hamas as the ruler of a postwar Gaza and how to bring back the scores of hostages still being held in the Palestinian enclave.

Mr. Gantz joined the government last October to foster a sense of unity at a time of crisis. He joined forces with his political rival, Mr. Netanyahu, despite a deep lack of trust between the two and a history of betrayal. The last time Mr. Gantz went into a government with Mr. Netanyahu, in 2020, it also ended badly after Mr. Netanyahu broke their power-sharing agreement.

The influence of Mr. Gantz and Mr. Eisenkot, whose son, a soldier, was killed in December while fighting in Gaza, has waned in recent months, leading many Israelis to ask why they had not left the emergency government and joined the opposition earlier. Mr. Gantz has called for early elections this fall.

Mr. Netanyahu’s formal partners remaining in the war cabinet are his defense minister, Yoav Gallant, a rival within his conservative Likud party whom Mr. Netanyahu tried to fire last year; and Ron Dermer, a seasoned Netanyahu confidant with more diplomatic than political experience. It is unclear if the war cabinet will continue to function.

A separate and broader security cabinet includes two ultranationalist party leaders: Itamar Ben-Gvir, the minister for national security, and Bezalel Smotrich, the finance minister. Both want to resettle Gaza with Israelis.

Mr. Ben-Gvir and Mr. Smotrich have both vowed to bring down Mr. Netanyahu’s government if he proceeds with an Israeli proposal for a deal involving a truce and a swap of hostages for Palestinian prisoners, which, as outlined by President Biden over a week ago, would effectively wind down the war.

At least two potentially destabilizing challenges now loom over Mr. Netanyahu’s government, analysts say.

The first is the prospect of a deal with Hamas. Israeli and American officials say they are waiting for a formal response from Hamas to the truce proposal. A positive response could well force Mr. Netanyahu to stop obfuscating and choose between a deal and the survival of his government.

The other challenge is the deeply polarizing issue of the wholesale exemptions from military service that are granted to ultra-Orthodox men enrolled in religious seminaries.

The ultra-Orthodox exemptions have long been a divisive issue in Israeli society, but tolerance for the decades-old policy has worn thin in a country where most 18-year-olds are drafted for years of compulsory military service, and even more so during this war. The same pool of reserve soldiers find themselves repeatedly called back for long stretches of duty in Gaza as the campaign grinds into a ninth month, with no clear plan, experts say, for where it is headed.

On Monday night or early Tuesday, the Israeli Parliament was expected to vote on a recruitment bill that would essentially keep the ultra-Orthodox exemption system intact. Though it is being pushed by Mr. Netanyahu to mollify his ultra-Orthodox coalition partners, even some members of his conservative Likud party — including Mr. Gallant, the defense minister — object to it, particularly during a war when the country needs more soldiers.

On the recruitment issue, Mr. Netanyahu finds himself in a bind, said Mr. Plesner. “There is an inherent conflict there between his own political base and his most precious alliance with the ultra-Orthodox parties,” he added.

If it passes this first reading, the bill will go into committee before the second and third, final, votes. But even if it fails to pass, said Mr. Plesner — who is himself a former lawmaker from a now-defunct centrist party — that won’t necessarily presage the dissolution of Parliament or collapse of the government.

Mr. Netanyahu’s critics accuse him of prolonging war to stave off elections and a public reckoning for the government and military failures leading up to the attack of Oct. 7.

Riffing off Mr. Netanyahu’s stated war goal of “absolute victory” over Hamas, which many experts say is a vague and unattainable notion, Mr. Gantz said in his resignation speech on Sunday that a “real victory” would be one that combined military success and diplomatic initiative.

“Real victory,” he said, means “changing national priorities, expanding the circle of service and those serving, and ensuring Israel is able to contend with the challenges it faces.”

“Unfortunately, Netanyahu is preventing us from reaching a real victory,” he added.

Mr. Netanyahu responded in a social media post addressing Mr. Gantz, saying, “Israel is in an existential war on several fronts. Benny, this is not the time to abandon the campaign — this is the time to join forces.”

Now, analysts say, Mr. Netanyahu is likely to be mainly focused on keeping his narrow coalition together for the short term.

The summer session of Parliament ends in late July and the legislature will only reconvene after the Jewish High Holy Days in late October or November.

“Netanyahu has only one thing in mind,” said Gayil Talshir, a political scientist at the Hebrew University of Jerusalem. “Maintaining his own power as prime minister.”

“His main aim is to drag this coalition just far enough into the autumn,” she said, so that the next Israeli election could only take place after the presidential election in the United States.

Mr. Netanyahu, she said, was likely hoping that Donald Trump, the candidate he views as most sympathetic to his causes, might then be elected.

That would mean that if he can get through the next six weeks, Mr. Netanyahu could live to fight another day.