Three years ago, Bernie Sanders’ quixotic campaign for the Democratic presidential nomination was garnering unexpected success.

But it was only in the wake of Hillary Clinton’s eventual defeat by Donald Trump that it became clear how much the Vermont senator had reshaped the Democrats’ political landscape.

As he mounts a second bid for the nomination, Mr Sanders’ left-wing policy positions — a steep hike in the minimum wage, free college tuition, a radical reform of the private healthcare system — are now widely accepted across much of the party, including by the Democrats competing against him for the chance to take on the president next year.

That impact, however, now appears to be spreading beyond the domestic front to foreign policy, including the party’s stance on the US-Israel relationship.

Last week, the Jewish senator defended Ilhan Omar, a Democrat congresswoman elected last November, against charges of antisemitism.

Ms Omar sparked anger last month by suggesting that US support for Israel was “all about the Benjamins”. The congresswoman apologised for that reference, which used a slang term for the $100 note, for appearing to evoke conspiracy theories about Jews buying political influence.

She caused further controversy two weeks ago by attacking “the political influence in this country that says it is OK to push for allegiance to a foreign country” — referring to Israel.

Eliot Engel, the Democrat chair of the House Foreign Affairs Committee, labelled those comments “a vile antisemitic slur”.

But although the House of Representatives passed a resolution late last week condemning antisemitism, it made no specific mention of Ms Omar and was widened — diluted, critics claimed — to include references to other forms of racism and hate speech.

Mr Sanders’ support of Ms Omar — in which he said moves to censure her were “wrong” and aimed at “stifling debate” — was echoed by some fellow Democrats running for the White House: Massachusetts senator Elizabeth Warren suggested that “branding criticism of Israel as automatically antisemitic has a chilling effect on our public discourse” while California’s Kamala Harris said the “spotlight being put on Congresswoman Omar may put her at risk”.

Other presidential candidates — such as New York senator Kirsten Gillibrand and Cory Booker, a member of the upper house representing New Jersey — took a harder line, but Mr Sanders’ sway with the liberal grassroots who will pick next year’s nominee is such that aspiring candidates may find it less politically costly to follow his lead.

Recent polls suggest that the 77-year-old senator leads the polls in New Hampshire, the first state to hold a primary election next year, and is fast gaining ground in Iowa, another early contest, where former Vice President Joe Biden — who has yet to declare — is currently ahead.

Mr Sanders was widely seen to have erred politically when he made a series of criticisms of Israel in the run-up to the New York primary in April 2016. Declaring himself “100 per cent pro-Israel”, he nonetheless said its actions in the 2014 Gaza war were “disproportionate”, inflated the number of Palestinian deaths and attacked Benjamin Netanyahu. Mrs Clinton went on to comfortably win the state, home to the largest Jewish population in America.

Buoyed by surveys showing rising sympathy among liberal Democrats for the Palestinians, Mr Sanders appears to have further toughened his stance towards Israel since 2016. He has floated cutting US military aid to the Jewish state and released a video last summer which labelled Gaza a “prison”.

The senator also denounced Mr Netanyahu as part of “a new authoritarian axis” alongside Mr Trump, Saudi crown prince Mohammed bin-Salman, Vladimir Putin, Hungary’s Viktor Orbán and the far-right Brazilian president Jair Bolsonaro.

Last month, five of the six Democrat senators running for president joined Mr Sanders in opposing an anti-BDS bill, claiming it violated the right to free speech.

Only Senator Amy Klobuchar, a midwestern moderate who had a rabbi attend the launch of her campaign earlier this month, backed the legislation.

To counter this drift, a new pro-Israel organisation, the Democratic Majority for Israel, was launched last month. It may soon have its work cut out.

While the Democratic leadership in Congress remains highly supportive of Israel, Ms Omar may be right in feeling she has, thanks to Mr Sanders, gained some powerful new friends.

As Bret Stephens of The New York Times described the controversy: “This is how progressivism becomes Corbynism.”