(CNN) Democrats are entering the 2020 election cycle with many of their leading presidential contenders increasingly willing to break with a pro-Israel foreign policy orthodoxy that guided the party for a generation.

The new crop of progressive political stars in the House and a base more sympathetic to the Palestinians than ever before helped push a half-dozen White House aspirants to break with the pro-Israel lobby last week on a major bill, even as it passed with support from Minority Leader Chuck Schumer and other high-ranking Democratic officials.

But the simmering divisions and debates inside the party blew up on social media Sunday night, when Minnesota Rep. Ilhan Omar suggested in a series of flippant tweets that the top House Republican's support for Israel was "all about the Benjamins" -- or tied to financial backing offered by the right-leaning American Israel Public Affairs Committee.

Omar's comments were denounced by members of both parties as "anti-Semitic" and, by Monday afternoon, she had responded to Democratic leadership's condemnation with an apology, saying in a statement posted online, "I unequivocally apologize." Earlier in the day, a pair of Democratic House newcomers sought to gather signatures for a letter rebuking Omar over her tweets and, in a move likely to escalate the controversy, her support for the Boycott, Divest and Sanctions movement -- known as BDS -- against Israel in response to its treatment of Palestinians.

"Such positions," Democratic Reps. Josh Gottheimer and Elaine Luria wrote, "are, at their core, anti-Semitic."

The blow-up threatens to further complicate an already knotty question facing progressives who have spent years now carefully questioning what they view as a political establishment that is too deferential and unwilling, over domestic political concerns, to criticize the Israeli government.

Contrary to Omar's claim, AIPAC does not donate to candidates. It does, however, spend millions lobbying elected officials -- a distinction she failed to make -- and does, as described on its own website , ask its would-be "Congressional Club" members to "commit to giving political contributions in a clearly pro-Israel context to candidates running for the United States House of Representatives and/or United States Senate."

"It's undeniable that money plays a huge role in our politics. Also true that tropes about Jewish money have been used for centuries to target Jews," Sen. Bernie Sanders' foreign policy adviser Matt Duss tweeted on Monday morning. "Progressives need to be extremely aware of that history and avoid those tropes as we talk together about how to fix our politics."

Sanders, who is Jewish, has been among the likely 2020 contenders most willing to denounce the Israeli government over its treatment of the Palestinians.

The ongoing political shift came clearly into focus last week when nearly all of the Democratic presidential hopefuls -- with Sen. Amy Klobuchar a notable exception -- voted against a Senate bill that would allow state and local governments to withhold contracts from those who participate in BDS. Most of them took the same position in explaining their opposition to the bill: They oppose efforts to boycott Israel, they said, but they feared the measure violated First Amendment rights.

"There are ways to combat BDS without compromising free speech, and this bill as it currently stands plainly misses the mark," New Jersey Sen. Cory Booker -- who had co-sponsored a similar bill as recently as November — said in a statement last week to Jewish Insider.

Most of the other Democratic presidential candidates and potential candidates in the Senate -- including Sanders, California Sen. Kamala Harris, New York Sen. Kirsten Gillibrand and Ohio Sen. Sherrod Brown -- opposed the legislation, all citing free speech concerns that overshadowed their opposition to the boycott efforts.

"I believe in free speech. And I do not support BDS but I think the steps to outlaw go too far," Massachusetts Sen. Elizabeth Warren told CNN recently.

The votes put the party's 2020 field largely at odds with Schumer, the New York Democrat, who backed it. They also failed to sink the effort which easily passed with support from both parties. It is unclear if House Democrats will bring up the bill for consideration.

But in voting in near lockstep against it, the presidential contenders highlighted a shift that has been gaining steam among Democrats for years.

In 2018, the Pew Research Center found that 27% of Democrats said their sympathies lied with Israel, while 25% said the Palestinians -- with the rest saying neither. That was a sharp drop from the 43% who chose Israel just two years earlier. Younger and more liberal Democrats, in particular, said they were more likely to side with the Palestinians. And some companies that cater to a younger clientele have also shifted their positions in response to Israeli government actions: Airbnb in November said it would stop listing rentals in West Bank settlements.

The election of two BDS supporters -- Omar and Michigan Rep. Rashida Tlaib, the first Muslim women in Congress -- to House Democratic ranks last year pushed the issue to the forefront.

"We are finally able to have conversations that we weren't really willing to," Omar told CNN recently, "and so it is really important for us to get a different lens about what peace in that region could look like and the kind of difficult conversations we need to have about allies."

Their willingness to criticize Israel, often in the context of clear and unabashed advocacy for Palestinians, has already begun to test old rhetorical boundaries in a way that even some of the party's more liberal elected officials have typically sought to avoid. Freshman lawmakers like Tlaib, Omar and New York Rep. Alexandria Ocasio-Cortez have been quick to reject those constraints, and seem poised now to insist on a conversation that top Democrats have long stifled or sought to avoid.

"Straight up, the party is not ready for the debate that is coming," one senior aide to a Democratic senator told CNN last year . "It is an overdue debate and my strong sense is that no, the party is not ready to have it -- look who the foreign policy leaders are. But that doesn't matter because it's coming."

A major moment that opened the doors for Democrats to become much more critical of Israeli policy came in 2015, when Benjamin Netanyahu came to Capitol Hill to denounce former President Barack Obama's Iran nuclear deal, said Jeremy Ben-Ami, the president of J Street, a left-leaning Jewish advocacy group.

"When he came to Congress and argued against the policies of the sitting President of the United States at the invitation of the Republican speaker at the time, that was really what I think will be regarded as the historical moment when that bipartisanship was shattered. And it opened up the possibility for people to have arguments over these issues in ways that they probably didn't prior to that," Ben-Ami told CNN last week. "It gave permission to people to actually say what they think. To open up."

Netanyahu "threw in his lot with these right wing dictators and with Trump and as a result the liberal Democrats that make up the majority of the Jewish community ... are going to oppose what Netanyahu is doing," he said.

In a statement on Monday, Ben-Ami described the controversy surrounding Omar's tweets and the storm of accusations of against her "overheated, ill-considered and reductive."

"Elected officials should be particularly sensitive and careful on the question of the role played by campaign contributions in influencing US policies toward Israel and the Middle East," he said, adding that they "should also refrain from labeling all criticism of Israeli actions or policies as 'anti-Semitic,' in a transparent effort to silence legitimate discussion and debate."

What's not yet clear is whether Democrats, including the party's new progressive vanguard, are willing to go any further in breaking with the Israeli government or curbing long-standing US support.

Republicans are eager to capitalize on what they characterize as Democrats abandoning Israel. House Minority Leader Kevin McCarthy, whose reported threat to "take action" against Omar and Tlaib over their criticism of Israel appears to have prompted Omar's tweets, is now accusing top Democrats of an "abdication of leadership."

McCarthy tweeted on Sunday that "Anti-Semitic tropes have no place in the halls of Congress," but before the midterms said in a post of his own, "We cannot allow (George) Soros, (Tom) Steyer, and (former New York Mayor Michael) Bloomberg to BUY this election! Get out and vote Republican November 6th. #MAGA" — a reference to three Jewish billionaire donors whose names have been used in anti-Semitic propaganda. He deleted the tweet a day later.

President Donald Trump, meanwhile, has embraced Netanyahu and pleased pro-Israeli advocates within both parties' ranks by moving the United States' embassy from Tel Aviv to Jerusalem. In the spring of 2017, a bipartisan anti-BDS bill arrived in the Senate with more than 50 cosponsors, another sign of how deeply entrenched support for Israel has become at the highest levels of national politics.

Despite the overwhelming initial support, Democratic Sen. Ben Cardin was pressured into redrafting parts of the legislation after it came under scrutiny by civil liberties groups. Gillibrand, an opponent of BDS initially listed as a sponsor of the bill, withdrew her name following a burst of criticism, led by the ACLU, and said she would ask the authors to "rewrite it" to make clear that individuals would not be targeted for taking part.

Grassroots Jewish progressive groups like IfNotNow, which was formed in 2014 during a conflict that became known by many as the Gaza war, believe opposition to Israel's presence in the West Bank, Gaza and East Jerusalem -- often described as "the occupation" -- fits in naturally with other, increasing popular priorities, many of them championed by Sanders.

"The same progressive values that so many Americans use to support issues like 'Medicare for all' or abolishing ICE and promoting more humane treatment of immigrants, raising the minimum wage, stricter gun control, keeping fossil fuel money out of politics are the same progressive values -- and they're Jewish values -- that lead us to oppose occupation," said Emma Saltzberg, an IfNotNow co-founder and volunteer organizer.