Left-wing parties in the U.S. and the United Kingdom are both struggling to deal with a wave of accusations of anti-Semitism within their parties -- as their bases push them increasingly to the left and toward positions more critical of Israel.

The controversy blew up for the Democratic Party this week when Rep. Ilhan Omar, D-Minn., suggested that supporters of Israel were pushing for U.S. politicians to declare "allegiance" to Israel.

HOUSE PASSES BROAD RESOLUTION CALLING OUT RACISM, 'ANTI-SEMITIC' COMMENTS -- WITHOUT NAMING ILHAN OMAR

“I want to talk about the political influence in this country that says it is OK for people to push for allegiance to a foreign country," Omar said. "I want to ask why is it OK for me to talk about the influence of the NRA, of fossil fuel industries, or big pharma, and not talk about a powerful lobbying movement that is influencing policy."

It was the latest controversial statement she’d made in reference to Israel that critics said used anti-Semitic dogwhistles. Omar tweeted in 2012: “Israel has hypnotized the world, may Allah awaken the people and help them see the evil doings of Israel."

Last month, she got into trouble when she suggested some members of Congress were being paid by the American Israel Public Affairs Committee (AIPAC) to support Israel.

“It’s all about the Benjamins baby,” she tweeted.

Omar apologized for those statements but has not done so for her “allegiance” comments, which ramped up pressure on Democrats to condemn her remarks -- leading ultimately to a resolution against bigotry that was voted on Friday and ultimately passed.

Meanwhile, in the U.K., the Labour Party -- which moved to the left after the election of once-fringe left-wing MP Jeremy Corbyn as leader in 2015 -- has been wrestling with a similar crisis of its own for years, but one that has dramatically escalated in recent weeks.

Corbyn is a longtime critic of Israel and has been criticized for his ties to extreme pro-Palestinian groups, including for a past meeting where he expressed support for members of Hamas and Hezbollah, who he called “friends.” He has since said he was using “inclusive language” as he sought to bring about a peace process and that he does not support or agree with them.

UK'S LEFT-WING LABOUR PARTY ENGULFED BY ANTI-SEMITISM CRISIS

Last year a picture emerged of him attending a wreath-laying ceremony in 2014 that honored perpetrators of the 1972 Munich terror attack against Israeli athletes, although he said he was there to honor those killed in a 1985 Israeli air strike.

Although he has repeatedly condemned anti-Semitism, Corbyn made things somewhat worse for himself when, in a 2016 speech intended to condemn anti-Semitism and distance himself from some of the controversies, he appeared to compare Israel to the Islamic State.

"Our Jewish friends are no more responsible for the actions of Israel or the Netanyahu government than our Muslim friends are for those of various self-styled Islamic states or organizations," he said.

But the party's pro-Corbyn left have turned those incidents into a full-blown crisis with a steady stream of anti-Israel and allegedly anti-Semitic comments.

A high-profile moment came in 2016 when top Corbyn ally and former London Mayor Ken Livingstone -- who is often referred to as “Red Ken” for his hard-left views -- was suspended by the party in 2016 for saying that Adolf Hitler was a Zionist before he “went mad and ended up killing 6 million Jews.”

Much of the controversy has focused on the failure of the party as a whole to appropriately discipline those accused of anti-Semitism. This week, the Times of London reported that a Corbyn ally recommended against the suspension of a Labour member who posted an image online of an alien with the Star of David on its back grasping the Statue of Liberty by the face, suggesting control over the US. The activist added the caption: “The most accurate photo I’ve seen all year!”

The link between a swing to the left -- including on Israel -- by parties and the rise of the anti-Semitism has been noted by both left-wing and right-wing commentators and outlets. In an explainer on the crisis, left-wing outlet Vox notes that the socialist left “tend to be passionately pro-Palestine” and that “such a position can lead to tensions between left-wing anti-Zionists and mainstream Jewish communities.

“This tension has at times led to a tendency on the left to indulge in anti-Semitic conspiracy theories and tropes -- like blaming a Jewish conspiracy for Western governments’ support of Israel or equating Jews who support Israel with Nazi collaborators.”

On the right, New York Times columnist Bret Stephens this week accused Omar of “bringing Corbynism to the Democratic Party” and called her a “case study in the ease with which strident criticism of Israel shades into anti-Semitism.”

“It says something about the progressive movement today that it has no trouble denouncing Republican racism, real and alleged, every day of the week but has so much trouble calling out a naked anti-Semite in its own ranks,” Stephens said.

“This is how progressivism becomes Corbynism. It’s how the left finds its own path toward legitimizing hate. It’s how self-declared anti-fascists develop their own forms of fascism.”

Nile Gardiner, director of the Heritage Foundation's Margaret Thatcher Center for Freedom, told Fox News that such ideas are not being challenged by leaders on the left as they once would have been.

"I think that what you're seeing is the rise of the far-left on both sides of the Atlantic and part of the far-left identity is a deep-seated anti-Semitism in both the U.K and the U.S., and there is a lack of willingness on the part of the political leaders to do anything about it," he said.

In the U.S., the Democrats struggled to condemn Omar for her remarks. Speaker Nancy Pelosi, D-Calif, was mocked for saying that Omar “has a different experience in the use of words." Meanwhile, Rep. James Clyburn, D-S.C., defended Omar, saying that her experience as a refugee from Somalia was "more personal" than Jews whose parents survived the Holocaust.

The resolution that passed Friday was initially intended to deal with Omar’s comments, but was eventually expanded to include all forms of bigotry including white supremacy. It was so broad that Omar was even able to hail it as a victory as it condemned anti-Muslim bigotry.

“Today is historic on many fronts," Omar, along with Reps. Rashida Tlaib, D-Mich., and Andre Carson, D-Ind., said in a statement. "It’s the first time we have voted on a resolution condemning anti-Muslim bigotry in our nation’s history."

“We are tremendously proud to be part of a body that has put forth a condemnation of all forms of bigotry including anti-Semitism, racism, and white supremacy," the statement continued. "At a time when extremism is on the rise, we must explicitly denounce religious intolerance of all kinds and acknowledge the pain felt by all communities. Our nation is having a difficult conversation and we believe this is great progress.”

In the U.K., the controversy is overshadowing Labour’s attempt to hold Prime Minister Theresa May’s feet to the fire over her handling of Brexit. A number of centrist Labour MPs broke off from the party last month and, alongside some Conservative Party rebels, formed the Independent Group -- an independent party bloc with an eye to becoming a centrist party. The Labour rebels cited the party’s handling of the anti-Semitism as a key reason for their departure and accused Corbyn of tolerating anti-Semitism in the Labour Party ranks.

“Over the past three years...the Labour Party under Jeremy Corbyn has become infected with the scourge of anti-Jewish racism,” a statement by MP Joan Ryan said. “This problem simply did not exist in the party before his election as leader. No previous Labour leader would have allowed this huge shame to befall the party.”

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It is unclear what such a breakaway group could mean for the next general election, which could come as early as this year. But one possibility is that it could split the left-wing vote, bringing in disenfranchised Labour centrists, giving an enormous electoral advantage for the Conservative Party due to the country's first-past-the-post electoral system.

Meanwhile, in the U.S., the controversy is already becoming an issue for potential 2020 Democratic presidential candidates. President Trump has latched onto the issue, saying it is “inconceivable” that Democrats “will not act to condemn [anti-Semitism].”

Sens. Bernie Sanders, I-Vt., and Elizabeth Warren, D-Mass., have backed Omar, but others have ducked the issue. Asked about Omar at a news conference Thursday, Sen. Cory Booker, D-N.J., insisted he wanted to hear only "on-topic" questions about unrelated legislation -- then, receiving none, stop taking questions entirely.

Gardiner, at the Heritage Foundation, said that the controversies could have electoral consequences in both countries: "I think most people in the U.K. and the U.S. fundamentally reject anti-Semitism, so it is a huge electoral liability and a real problem for the left on both sides of the Atlantic, this sort of growing tolerance by left-wing politicians of anti-Semitic statements and behavior, because that is not where the British or the American people are."

Fox News' Gregg Re contributed to this report.