New York (AFP) - When Bernie Sanders called Israel's response in the 2014 Gaza war disproportionate and urged America to be more balanced on the Israeli-Palestinian conflict, he smashed a presidential campaign taboo.

His remarks at the April 14 Democratic debate ahead of New York's decisive primary on Tuesday amounted to unprecedented criticism of Israel and promotion of Palestinian rights from a canvassing US presidential candidate.

Israeli Prime Minister Benjamin Netanyahu "is not right all of the time," said the Vermont senator. "We cannot continue to be one-sided."

He criticised Democratic frontrunner Hillary Clinton for not saying that she would do more to promote Palestinian rights when she addressed the powerful right-wing American Israel Public Affairs Committee (Aipac) lobby in Washington in March.

"If we are ever going to bring peace to that region, which has seen so much hatred and so much war, we are going to have to treat the Palestinian people with respect and dignity," Mr Sanders said.

Mr Sanders, who is the only Jewish presidential hopeful this year, has lived in Israel.

In other countries the remarks would have been considered run of the mill, said Mr Daniel Sieradski, national organiser of the group "Jews for Bernie" which has 8,000 supporters on Facebook.

"But because the discourse in American Jewish politics has been pulled so far to the right in the last couple of decades, Bernie is being made to sound like some anti-Israel extremist."

During the last contested Democratic New York primary in 1992, it might have been political suicide, The New York Times wrote.

Mr Sieradski disagreed. "I don't think it's political suicide," he told AFP. "But it definitely didn't help him among people who have hard-line views on Israel."

He said Jewish Americans make up 20 percent of the New York electorate. The majority of them are Democrats, meaning that air-tight support for Israel has long been considered a campaign must.

But Mr Sanders' words reflect changing attitudes among Jewish voters, particularly Millennials who grew up seeing Israel as a strong state and for whom the horrors of the Holocaust are more removed.

Democrat Sharon Goldtzvik, 29, told AFP she was "really excited" to see a presidential candidate bring up the issue of Palestinian dignity.

She founded and runs Uprise, a communications firm which works with non-profit groups and focuses on human rights and the Middle East. Ms Goldtzvik has lived in Israel, is married to an Israeli, and describes Mr Sanders as "a breath of fresh air."

"I'm under 30. People in my cohort were not willing to accept (that) there is only one way to support Israel, so I do think that he represents the views of many, many Jews and a growing number of Jews."

According to a Pew Research Center poll, 35 per cent of Democrats thought Israel had gone too far in its response during the 2014 Gaza.

Mr Sanders has "at least opened up the discourse so the conversation can shift in the Democratic Party, and that's a big deal," said Mr Sieradski.

Polls show that Mr Sanders trails Mrs Clinton 40-60 per cent among Jewish Democrats in New York City, and 13 points behind his opponent on a state-wide average.

Documentary filmmaker Gaylen Ross is voting for Clinton and believes she is the candidate best able to negotiate a two-state solution.

"Frankly if that's the kind of language that he comes to a negotiating table with he is already 10 steps behind," she told AFP.

"You don't play your hand before you get to the table and you don't play your hand before you get to a national election."

Mr Sanders' suspension of his Jewish outreach director for referring to Mr Netanyahu in vulgar terms also signals a lack of experience or suggests he is not informed, Ms Ross said.

Mr Sanders was the only the candidate who declined to speak at the Aipac event in Washington on March 21. He also told the New York Daily News - mistakenly - that he thought more than 10,000 civilians were killed in Gaza in 2014.

Mr Howard Graubard, a New York lawyer active in Democratic politics in the state, does not expect Mr Sanders to suffer much at the ballot box "because he was going to lose anyway." It won't alienate his progressive Jewish supporters, but his criticism of Israel gives Orthodox and right-wing Zionist Democrats, who feel little enthusiasm for Clinton, a reason to get out and vote.

"They're nominal Democrats and need a motivator," Mr Graubard said.

"People are being emailed, fliers will be going up this weekend, there will be phone banks, and the message about Bernie and Israel is going out to pull those people out to vote."