Regardless of which party wins control of Congress in November, New York City voters have all but ensured that next year a vocal bloc of new House Democrats will arrive on Capitol Hill, elected, in large part, because they believe Israel has committed a genocide in Gaza.

That is an upshot of the primary elections held Tuesday in New York, where voters ousted two incumbent House Democrats and replaced a third who is retiring with progressives championed by Zohran Mamdani.

The democratic socialist, who is the city’s first Muslim mayor, campaigned on opposing US support for Israel and rejecting the influence of the American Israel Public Affairs Committee (Aipac), which has for decades donated heavily to candidates that ally with the country’s government.

The trio of insurgent victories in New York has sent shockwaves through the Democratic establishment, underscoring how internal divisions over Israel – an issue that dogged Joe Biden – continue to shape races across the country ahead of November’s midterm elections, when the party hopes to win back power from Donald Trump’s Republican allies.

“I think that it is a testament to just how much our movement has progressed against lobbies like Aipac, who have tried to spend hundreds of millions of dollars to keep us in a position where we spend hundreds of billions of dollars of our taxpayer money to fund a genocide and weapons and bombs abroad,” said Usamah Andrabi, a spokesman for Justice Democrats.

The progressive group had recruited doctoral student Darializa Avila Chevalier, who narrowly beat five-term congressman Adriano Espaillat after criticizing him for accepting money from Aipac. In the district represented by retiring congresswoman Nydia Velázquez, Justice Democrats endorsed state assembly member Claire Valdez, who triumphed in the Democratic primary over Brooklyn borough president Antonio Reynoso, after convincing voters she was more opposed to Israel than he was.

“Now, we are seeing what happens when you give voters the opportunity and candidates who not only reject Aipac money and call it out for what it is – a rightwing lobby – but also uncompromisingly stand up to oppose continued funding for the genocide and for a free Palestine,” Andrabi said.

More than 75,000 people have been killed in Gaza since October 2023, after Hamas militants killed some 1,200 and took 250 hostage in an attack on Israel. Biden’s decision to back prime minister Benjamin Netanyahu in the wake of the 7 October assault led to him being dogged by protesters who decried him as “Genocide Joe”, and may have dampened enthusiasm for Kamala Harris’s failed presidential campaign.

A ceasefire in the conflict was reached last October, but the question of whether to call Israel’s actions in Gaza a genocide – a determination reached by human rights groups and a United Nations commission – has roiled Democratic primaries across the country and turned into something of a litmus test for the left.

As he faced off for the Democratic nomination against New York comptroller Brad Lander, congressman Dan Goldman sought to downplay the conflict’s significance to his voters, while objecting to the use of the word “occupation” to describe Israel’s military presence in Gaza and arguing its campaign did not amount to a genocide.

“Israel is not the most important issue in this district,” Goldman said, during a primary debate. He wound up losing to Lander by more than 30 percentage points.

“Democrats are painfully divided by our differences over the US relationship to Israel and Palestine,” Lander said in his victory speech on Tuesday night. “We have to face up to it squarely.”

Lander said he intended to be “one of the Jewish members of Congress most willing to stand up for Palestinian human rights” and urged the party to “admit that Joe Biden’s hug-Bibi strategy was a catastrophic failure”.

“I believe it made us complicit in genocide,” he told supporters.

After comparing the two Democrats, Manny Fidel, 34, a writer based in Brooklyn, said he ended up supporting Lander because of his views on Israel and Palestine, which he described as “the big issue of this race”.

“I liked how kind of clear-headed Brad is on that issue,” Fidel said. “It’s become kind of a litmus test for national elections here in the United States – and I think it should be. I think it’s the huge moral issue of our time.”

Corbin Trent, a former aide to Democratic congresswoman Alexandria Ocasio-Cortez, said US involvement in the conflict in Gaza was similar to its 2003 invasion of Iraq, in that it woke up scores of voters to the consequences of Washington’s actions overseas.

“Our foreign policy, generally speaking, has been something that when people are confronted with the cost of that in lives abroad, and lives here, and in money, they reject it,” Trent said. “That was one of the things that propelled Maga, that’s one of the things that propelled these victories here.”

The rise of Avila Chevalier, who defeated Espaillat, the current chair of the Congressional Hispanic Caucus, has had perhaps the most disruptive impact on her party.

She had made social media posts criticizing Biden and Harris but has since repudiated them. Following the primary, former Democratic National Committee chair Jaime Harrison wrote on social media: “I say this with no ill will or animosity: if you hate the Democratic party, then please don’t run for our nomination.”

Tom Malinowski, a former congressman who recently lost his bid to return to the House to a progressive, criticized Mamdani and Sanders for endorsing Avila Chevalier, warning that they risked creating “the mirror image of Maga”.

“It doesn’t matter if candidates are qualified or what their beliefs are. All that matters is that they’re on the team, backed by its leader. Anyone who questions that is slated for the next political destruction,” Malinowski wrote.

He added: “When your followers treat Israel as the world’s only human rights violator and Aipac as the only evil dark money group, that should be a red flag.”

Avila Chevalier, Valdez and Lander are seen as locks to win House seats in the November general election, since their districts are heavily Democratic. Congressional Republicans pounced on their ascendancy, saying it was proof that “communists” had taken over the Democratic party.

“The Democratic party is in big trouble,” Trump said, speaking on Friday at the Faith & Freedom Coalition’s annual Road to Majority conference in Washington. “This is not stopping with New York.”

The trend may continue when Colorado holds primaries on Tuesday, and 15-term congresswoman Diana DeGette will face a challenge for the Democratic nomination from Melat Kiros, a democratic socialist who has criticized the incumbent for being too supportive of Israel.

This issue also looms in Michigan, where Democrats are hoping to keep hold of the Senate seat being vacated by Gary Peters, who is retiring. Among the leading Democratic candidates, Aipac has backed congresswoman Haley Stevens, while state senator Mallory McMorrow and former public health official Abdul El-Sayed have both accused Israel of committing “genocide” in Gaza.

And it is almost certain to be an issue in the presidential election in 2028, when voters nationwide will choose a successor to Trump.

“Every candidate for the presidency now on the Democratic side will be required to declare himself or herself on the matter of the United States’ stance towards Israel,” predicted Bill Galston, a former domestic policy adviser to Democratic president Bill Clinton who is now a senior fellow in governance studies at the Brookings Institution thinktank in Washington.

“The question was largely evaded in 2024. That strategy is no longer possible.”

David Smith in Washington, Adam Gabbatt in New York and Lauren Gambino in Los Angeles contributed reporting