Six years ago this week, one of the most dramatic, historic and contentious battles for the Democratic party’s presidential nomination drew to a close.

After contests in 50 states, countless forums and TV debates, and having between them racked up $430m in spending, Senator Bernie Sanders – who had begun his quixotic campaign a year earlier nearly 50 points adrift in the polls – had fought the long-time favourite, Hillary Clinton, right down to the home straight.

While Mrs Clinton – the first major-party female nominee and an unapologetic centrist – was the clear choice of the Washington establishment, Mr Sanders, a self-described socialist, revelled in the role of political outsider. And, when he trounced Mrs Clinton by a whopping 22 points in the much-watched New Hampshire primary, Mr Sanders became the first Jew to win a presidential primary; he’d eventually go on to beat her in another 21 states.

The length of the contest disguised the fact that, overall, Mrs Clinton bested Mr Sanders by an impressive 12 points or nearly four million votes. Still, after Mrs Clinton’s defeat by Donald Trump five months later, Mr Sanders became the frontrunner for Democrats’ presidential nomination in 2020 and one of the most influential figures in charting its future course. Not bad for a man who, as the longest-serving independent in US congressional history, isn’t even formally a Democrat.

But Mr Sanders now appears to view himself not simply as a major player in the party, but its moral compass and the arbiter of the boundaries of political acceptability.

Late last month, for instance, he launched a blistering attack on AIPAC and its super PAC, the United Democracy Project, which has been spending heavily in support of pro-Israel candidates in Democratic party primaries.

Mr Sanders didn’t engage in the substance of his disagreements with AIPAC. Instead, he told the New York Times, the battle against the lobbying group was nothing less than “a war for the future of the Democratic Party”. AIPAC’s goal, he continued, was “to create a two-party system, Democrats and Republicans, in which both parties are responsive to the needs of corporate America and the billionaire class.” “They are doing everything they can to destroy the progressive movement in this country,” railed the Vermont senator.

It's true that, as Mr Sanders said, AIPAC is a bipartisan organisation which does nothing to hide its aim of electing supporters of the Jewish state on both sides of the aisle. He’s also right that a group which proudly touts Israel as the Middle East’s sole democracy shouldn’t be donating a penny to Republicans who – by refusing to certify the outcome of the 2020 presidential election – did their utmost to undermine the rule of law and the democratic process in the United States itself.

But Mr Sanders’ histrionic and sanctimonious rhetoric underlines his view of politics – one shared by Mr Trump and his supporters – not as a forum for compromise and civil disagreement, but as a battlefield in which opponents are enemies with malign and irredeemable motives.

It is, in fact, Mr Sanders’ worldview and his desire to yank the party to the hard left – not AIPAC’s decision to fight back against those who obsessively seek to demonise and delegitimise the world’s sole Jewish state – which represents the bigger threat to the electoral viability of the Democrats. Ironically, this is epitomised by Mr Sanders’ seemingly careless attitude towards one of the party’s most loyal constituencies: American Jews.

Consider, for instance, the contrast between Mr Sanders’ harsh treatment of Mrs Clinton and the manner in which he has indulged the hard-left Congresswoman Ilhan Omar.

Even as it became increasingly apparent that Mr Sanders couldn’t – and wouldn’t – defeat Mrs Clinton for the nomination, he continued to wage a war of political attrition against her – despite the fact that, by this stage, it was also quite clear that she would be going up against a perhaps uniquely dangerous and flawed Republican opponent in November.

In What Happened – her brutally honest, and often self-lacerating, account of her defeat in 2016 – Mrs Clinton accurately depicts Mr Sanders of going beyond the normal political knockabout of an intra-party contest. Accusing him of “innuendo and impugning my character”, Mrs Clinton suggests that Mr Sanders’ attacks “caused lasting damage, making it harder to unify progressives in the general election and paving the way for Trump’s ‘Crooked Hillary’ campaign”. She takes issue, too, with Mr Sanders’ supporters — “the so-called Bernie Bros” — for “harassing” her supporters online.

Mr Sanders, however, adopted a rather more indulgent attitude when Mrs Omar – a relentless critic of Israel who supports the BDS movement – opted to begin her congressional career by spouting a series of offensive tropes. In February 2019, for instance, she suggested that support for Israel among US politicians was “all about the Benjamins” – a slang term for the $100 note which appeared to evoke conspiracy theories about Jews buying political influence. (Mrs Omar later apologised and deleted the tweet). She caused further controversy two weeks later by attacking, in a clear reference to Israel, “the political influence in this country that says it is OK to push for allegiance to a foreign country”.

Mr Sanders, however, argued that talk of censuring her in the House of Representatives was “wrong” and aimed at “stifling … debate” and – while mildly conceding that the Minnesota Democrat “maybe” needed to do “a better job in speaking to the Jewish community” – also said he wanted to “support a Muslim member of Congress not to be attacked every day in outrageous, racist remarks”.

Still, having suggested at the time of the controversy that he had “talked to Ilhan about twice in my life”, by the time Mr Sanders had once again hit the presidential campaign trail a little under a year later, he was hailing her as “one of the greatest people I know”.

For moderate Democrats such as Mrs Clinton, however, Mr Sanders has had no such kind words to offer. Indeed, his anger at two conservative Democrat senators – West Virginia’s Joe Manchin and Arizona’s Kyrsten Sinema – who have frustrated elements of Joe Biden’s legislative agenda led him to suggest in January that he’d be “happy to support” primary challenges against both of them in 2024. The fact that, as of March 2022, Mr Manchin had voted in line with Mr Biden’s positions 95.5 percent of the time – two points higher than Mr Sanders’ own score – and Mrs Sinema 97.7 percent of the time seemed to pass him by. So, too, did the fact that Mr Trump had won West Virginia by nearly 40 points in 2020 and that any replacement for Mr Manchin was unlikely to be the liberal Democrat of Mr Sanders’ dreams but a conservative Republican.

Despite Mr Sanders proclaiming his pride in Israel and opposing BDS, Mrs Omar’s fulsome backing wasn’t the only questionable endorsement he was willing to accept.

Perhaps most controversially, his campaign picked far-left Israel critic Linda Sarsour to become a surrogate and touted her support on twitter. Similarly, Mr Sanders said he was “humbled” to receive the official endorsement of the United Electrical, Radio, and Machine Workers of America union, despite it being the first American trade union to back the BDS movement in 2015. Mr Sanders’ stance led to a public ticking off from the American Jewish Congress, which noted that “with the tide of anti-Semitism rising worldwide, it is of utmost importance that Jewish political and community leaders, such as yourself, show Jewish Americans that they will stand up for them.”

However, none of this appears to have led Mr Sanders to adopt a more careful, balanced position on Israel. Instead, the strident and vocal attacks on Israel by members of the “The Squad” of hard-left congresswomen appears to have, in the words of the AJC president Jack Rosen, “emboldened” Mr Sanders. In a radical departure from the Democrats’ own long-standing position, he pushed for conditioning military aid to Israel, helping shift the terms of the debate during the party’s primaries and subsequently.

All of this, of course, has the ring of familiarity for British observers. Unsurprisingly, his media cheerleaders hailed Jeremy Corbyn’s Labour as a “model for US progressives”. But the “Corbynesque Sanders”, as one admirer labelled him, needed no convincing. As Mr Sanders said of Mr Corbyn on the eve of the 2017 general election: “What has impressed me – and there is a real similarity between what he has done and what I did – is he has taken on the establishment of the Labour party, he has gone to the grassroots and he has tried to transform that party … and that is exactly what I am trying to do.” Thankfully, Mr Sanders hasn’t – yet – achieved that goal.