Among the Jewish parties backing Mr. Gantz was the seven-seat, ultranationalist party Yisrael Beiteinu, led by Avigdor Liberman, a onetime ally of Mr. Netanyahu who has made it his mission to end the prime minister’s career.

Both Mr. Liberman and leaders of the Joint List had kept their intentions in doubt right up until their meetings with Mr. Rivlin.

The most surprising support for Mr. Gantz came from the three lawmakers belonging to Balad, an Arab-nationalist faction in the Joint List that opposes the idea of Israel as a Jewish state. Mtanes Shehadeh, the party’s leader, said Balad had initially planned to abstain from recommending Mr. Gantz, but endorsed him “in order to topple the leadership of the largest generators of incitement against the Arab society and its leadership.”

Mr. Gantz remains a great distance away from ousting Mr. Netanyahu. His ability to play hardball politics is constrained by the virus emergency and by an almost universal public aversion to a fourth election. And he will need to hold together his motley collection of supporters, who are anything but natural allies, in the face of an anticipated onslaught from Mr. Netanyahu and his defenders.

Still, in contrast to last fall, when he and Mr. Netanyahu each took a turn trying to form a government without success, Mr. Gantz holds important new advantages.

Crucially, he now has the backing of an absolute parliamentary majority, however thin. That gives Mr. Gantz the initiative, and his supporters say Mr. Netanyahu will have to either acquiesce to Mr. Gantz’s becoming prime minister or stand in the way — at a time when the public health emergency has created grave political risks for any lawmaker seen as obstructing the response to it.