The Arab bloc in Israel’s parliament abandoned its usual hands-off stance Sunday and endorsed former military chief Benny Gantz for prime minister, potentially giving him the edge over hard-line incumbent Benjamin Netanyahu.

The historic move marked the first time in nearly three decades that the Arab parties backed a candidate for prime minster, reflecting their contempt for Mr. Netanyahu, who was accused of fomenting hatred of the Arabs during his re-election campaign.

“Benny Gantz is not our cup of tea,” said Arab lawmaker Ahmad Tibi. “But we promised our constituents that we would do everything to topple Netanyahu, and the default here is recommending Benny Gantz.”

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It will be up to Israeli President Reuven Rivlin to decide which candidate should be given the chance to form a coalition government and serve as prime minister — a usually pro=forma task made difficult this time by last week’s deadlocked parliamentary elections.

Neither Mr. Gantz, the leader of the centrist Blue and White party, nor Mr. Netanyahu, head of the conservative Likud movement, has a majority in the 120-seat Knesset, or 61 members. But with the backing of the Arab parties, Mr. Gantz gained a slight advantage. He could have as many as 57 backers, compared with Mr. Netanyahu’s 55.

Maverick politician Avigdor Lieberman, head of the mid-size Yisrael Beitenu party, controls the final eight seats, making him the key powerbroker. But he announced Sunday that he isn’t endorsing either candidate.

The endorsement by the Joint List, a bloc of four small Arab parties that controls 13 seats, marked a turning point in Israeli politics. Arab parties have traditionally refrained from endorsing a candidate for prime minister, and they have never sat in a coalition government, not wanting to be seen as legitimizing Israeli policies.

Although many Israeli Arabs remain angry at Mr. Gantz for leading the military’s devastating war against Gaza militants in 2014, their fury toward Mr. Netanyahu runs much deeper. Mr. Netanyahu’s campaign repeatedly accused the Arabs of trying to “steal” the election, drawing accusations of racism and incitement.

There is also deep-seated anger over a law passed by Mr. Netanyahu’s government in 2018 that declared Israel to be the nation-state of the Jewish people. Arabs believe the law codifies discrimination.

Though the Arab parties are still expected to stay out of Mr. Gantz’s future government, their endorsement reflects a growing desire of Israel’s large Arab minority to take a more active role in shaping the country.

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In an op-ed piece in the New York Times, Ayman Odeh, the leader of the Joint List, argued that his move should end Mr. Netanyahu’s political career and provide a watershed moment for Israel’s Arab minority.

“If the centre-left parties of Israel believe that Arab Palestinian citizens have a place in this country, they must accept that we have a place in its politics,” he wrote. “There is no shared future without the full and equal participation of Arab Palestinian citizens.”

Mr. Netanyahu reacted by saying: “As we warned, the Arab parties that oppose Israel as a Jewish and democratic state and glorify terrorists recommended Gantz for prime minister.”

The Arab parties’ decision does not guarantee Mr. Gantz will be prime minister. The President usually appoints the candidate with the most supporters, but not necessarily. Mr. Rivlin has wide discretion.

In last week’s vote, Blue and White won 33 seats, while Likud took 31.

The deciding factor looks to be Mr. Lieberman, who is demanding a broad unity government with the two major parties that will be secular and exclude the ultra-Orthodox Jewish parties and Arabs. That appears to be the compromise emerging between Blue and White and Likud, though both are insisting upon leading it.

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Complicating matters is Blue and White’s refusal to sit with Mr. Netanyahu because he faces a likely indictment on corruption charges.

Mr. Odeh said he is planning on becoming opposition leader in the case of a unity government. As the leader of the largest party in opposition, he would be entitled to the position, which includes meetings with visiting world leaders and access to sensitive security information.

Mr. Rivlin has not indicated which way he is leading. But he said he interpreted the will of the people as yearning for a “stable” government. “And there can’t be a stable government without the two big parties,” the 80-year-old President said.

Last week’s vote was Israel’s second election in five months. Mr. Netanyahu was unable to form a coalition after April’s balloting.

In calling a new election, Mr. Netanyahu had hoped to secure a narrow majority of hard-line and religious parties that would grant him immunity from prosecution on charges that could include bribery, breach of trust and fraud. But now that possibility appears to be off the table.

Israeli law does not require a sitting premier to resign if indicted. But if he is charged, as is widely expected, he will come under heavy pressure to step down.