Four months before the US midterm elections, a rising challenge confronts the Democratic Party.

Israel, and its military campaign in Gaza and the wider Middle East, is fast becoming one of the party’s defining issues and could play a role in reshaping the Democratic Party’s leadership in future elections.

The evidence so far from the 2026 Democratic primaries, in which voters pick the candidate who will represent the party in November, suggests this could become a wedge issue between the base and the leadership.

The results of last week’s New York City congressional primaries were the clearest example yet and these divisions were particularly evident in the race to represent the city’s 10th congressional district.

While both Democratic candidates were Jewish, opposed President Donald Trump’s agenda, and identified as liberal Zionists, one had a markedly tougher stance on Israel than the other.

City Comptroller Brad Lander, who has called Israel’s military campaign in Gaza a genocide, pledged to vote against US funding for Israel’s military, and said he would not accept campaign donations from the American Israel Public Affairs Committee (AIPAC) lobbying group.

Meanwhile, Congressman Dan Goldman, who has also criticised the Israeli government and opposed the expansion of Israeli settlements in the West Bank, did not use the term genocide to refer to Gaza, and had previously received contributions from AIPAC.

Goldman spoke frequently on antisemitism and received backing from some community leaders in the Orthodox Jewish community, which is a prominent voting base.

Last week, Lander’s approach prevailed. He defeated Goldman in a landslide.

The results in New York’s 10th, as well as two other primaries in the city featuring progressives backed by New York City Mayor Zohran Mamdani, showed that candidates who made criticism of Israel central to their policy platforms prevailed over candidates that voters perceived as more moderate.

It hinted at a schism in the Democratic Party that could impact upcoming races this autumn, including a closely-watched Democratic primary for Michigan’s US Senate seat in August.

Because New York City’s voting base is predominantly Democratic, their primary winners will likely enter Congress next year. Their presence in Washington could force tougher policy debates over US funding for Israel and its military, and reshape the broader party’s messaging about Israel and Gaza.

“I think what we’re seeing is that in much of the Democratic base – especially in urban, progressive, young environments – criticising Israel publicly is sort of a signal that a candidate is not going to stick to the status quo, that they’re going to Congress to fix things up,” said Tali DeGroot, vice-president of political and digital strategy at J-Street, an organisation that supports Israel but has criticised Netanyahu and the Israeli military actions in Gaza.

As American attitudes shift, voters – especially young voters who identified with Democrats – “want to see elected officials engaged in a nuance way in a complicated issue,” de Groot said.

J-Street endorsed Goldman in the race but in an unusual move also added Lander as an “approved” candidate for voters to consider. Lander also received the endorsement of Jews for Racial and Economic Justice (JFREJ), a left-leaning social justice group that has called for a ceasefire in Gaza and opposes the sale of US weapons to Israel.

At polling locations in Brooklyn last Tuesday, Lander’s voters cited his critical approach to Israel and his endorsement by Mamdani as crucial factors in casting their ballots. The mayor has also been a vocal critic of Israel.

Lander’s voters said their party’s leadership was out of touch with its base of voters on the issue.

“I learned a lot watching a Jewish leader walk through his relationship to Zionism, to Israel, and to opposing genocide in Gaza,” said Perrin Roosevelt Ireland, who voted for Lander in the Park Slope neighbourhood of Brooklyn on Tuesday. “Watching his journey has been really powerful to me, and I like a leader that learns in public.”

She believed Senate Majority Leader Chuck Schumer and House Minority Leader Hakeem Jeffries – Congress’ two top Democrats who also represented New Yorkers – were out of touch with many of their constituents in their broad support of Israel.

“I think they’re really missing something,” she said.

The Israeli military launched a campaign in Gaza in response to the unprecedented Hamas-led attack on southern Israel on 7 October 2023, in which about 1,200 people were killed and 251 were taken hostage.

At least 73,058 people have been killed in Israeli attacks in Gaza since then, including more than 21,280 children, according to the territory’s Hamas-run health ministry, whose figures are seen as reliable by the UN.

Israel has strongly denied the accusations of genocide which have been made by a number of international and Israeli human rights organisations, independent UN experts and scholars.

Last week a report by a United Nations commission of inquiry said Israeli authorities and security forces had deliberately targeted Palestinian children in Gaza, resulting in genocide, crimes against humanity and war crimes. The three-member expert panel does not officially speak for the UN.

In response, Israel’s foreign ministry said it “utterly rejects” the commission’s report, calling it libellous propaganda.

A Pew Research Center survey in April suggested that 60% of US adults had an unfavourable view of Israel, external, up from 53% last year.

Eighty percent of Democrats and Democratic-leaning voters shared that view, an 11 point increase from last year.

This shift in American sentiment has also hit the Republican Party, with more than half of Republicans under 50 now viewing Israel unfavourably, according to Pew. Trump himself has expressed exasperation with Netanyahu and the “America First” isolationist wing of the party has been vocal in its displeasure at the alliance.

“At first, American opinion was very sympathetic toward Israel,” said Dina Smeltz, managing director at the Chicago Council on Global Affairs of Public Opinion and Foreign Policy.

“But then as it retaliated against those attacks and carried out a lot of military action in Gaza that resulted in a big humanitarian crisis, attitudes toward Israel have fallen quite a bit.”

LISTEN: Is the US falling out of love with Israel?

As public opinion within the Democratic Party base shifts, some voters and Jewish groups have expressed discomfort with the implications.

Democratic Jewish Council of America CEO Haile Soifer expressed “regret that Israel became a divisive issue in these races, especially in NY-10”.

Brooklynite Rob Geaillig told the BBC he voted Goldman because he believed Lander “is far too liberal for this area”. Though registered as a Democrat, he voted for Donald Trump in 2024 because he believed his party was too far “left” on issues such as Israel.

And other New York incumbents who have voted or spoken in support of Israel won their races.

Mainstream Democrats, including party leaders such as Schumer and Jeffries, have walked a careful line of criticising Israel’s military action in Gaza and Prime Minister Benjamin Netanyahu’s leadership, while continuing to broadly support Israel and legislation that benefits the country.

There are signs the Democratic congressional caucus is becoming tougher on Israel. In April, 40 Democratic senators voted to express “congressional disapproval” of proposed military sales to Israel. Only seven Democratic senators – including Schumer – voted with Republicans to block the motion. Though the vote was primarily symbolic, it indicated lawmakers’ growing frustration with ongoing military actions.

Sam Geisler, a 37-year-old Lander voter, said that Democrats who continued to vote to provide military aid to Israel could face upcoming challenges in other elections.

“We’ll see if the national party learns. I don’t have a whole lot of faith,” Geisler said. “They’re going to learn when they keep getting voted out.”

Asma Khalid contributed to this report