Bernie Sanders, a long-standing senator from Vermont, is the only “democratic socialist” in the race to become US president. Though this week’s influential Iowa caucus now faces a recount, initial results had him neck-and-neck with Pete Buttigieg – and he could yet surge past the 38-year-old to win the Democrat nomination.

Sanders’ promise of a political “revolution” in America driven by grassroots campaigning appears to excite younger left-leaning voters but gives establishment Democrats the jitters, sentiments that have grown since the 78-year-old started topping the polls to be the party’s nominee. His ascendency even came after a heart attack in October.

From unknown to household name

The one-time mayor of Burlington, Vermont, went from a national unknown to a household name after the 2016 Democrat presidential contest when he unexpectedly gave Hillary Clinton a close run for the nomination. His people-powered campaign was driven by the slogan “Feel the Bern” and marked by his ability to attract voters disenfranchised by mainstream politics.

Surveys have since crowned Sanders the most popular politician in the US, adding weight to the argument he would have beaten Donald Trump four years ago. Tensions between supporters of Sanders and Clinton continue to resurface as the wounds inflicted during a bruising campaign still fester.

Not technically a Democrat

Sanders is the longest-serving independent member of Congress in American history, having first been elected to the US House of Representatives in 1990 and the Senate 16 years later. He is one of two independents who have been admitted to the Democratic Caucus of the Senate, meaning he effectively serves the party in the upper house.

Why is party affiliation important? Sanders supporters have accused the ruling Democratic National Committee (DNC) of conspiring to thwart both his presidential runs and pave the way for a more “establishment” candidate. This week’s botched Iowa caucus has revived claims about the party’s attempts to “rig” the system against Sanders.

His policies are to the left 

Sanders is running on a platform of left-wing policies – or, at least, left-wing by American standards – which sets him apart from his centrist rivals. His promises include passing a jobs-boosting Green New Deal and increasing the minimum wage, policies that are anathema to Republicans but are pretty mainstream ideas in Britain.

Most notable is a promise to introduce an NHS-style, free-at-the-point-of-care healthcare system, which is referred to as “medicare for all”. It would mean abolishing the private healthcare system that, he argues, leaves millions either denied access to doctors or face crippling bills.

The type of state-run care he proposes is so embedded in European politics that even right-wing governments over here do not want to dismantle it. However, it is so unfamiliar to US voters that even some Democrats actively oppose what Sanders is proposing.

Trump, who refers to his potential rival as “crazy Bernie”, warned of a “socialist takeover” of healthcare in his State of the Union address this week, and is probably more than happy his media outriders deliberately conflate the term with the far more provocative “communism”.

Sanders himself likens his approach to a continuation of Franklin D Roosevelt’s New Deal, the massive welfare and building programmes that helped revive the country during the 1930s Great Depression. “This is the unfinished business of the Democratic Party and the vision we must accomplish,” he said in a speech last year. It’s the pro-worker, anti-big business approach at the heart of these policies that many think can see Sanders defeat Trump, particularly in the crucial “rustbelt” Midwestern states.

Some have pointed to Jeremy Corbyn’s election failure in the December as proof that “radicals” are unelectable. But Sanders is seen as a more pragmatic politician with actual administrative experience, unlike Corbyn the eternal campaigner. And while Sanders will often refer to his opposition to the Iraq war, in contrast to rivals including Joe Biden, he is less obsessed with foreign policy than the UK Labour Party leader and more concerned with how politics practically affects people’s daily lives.

A committed and unconventional following

Perhaps the best known of his supporters is the wildly popular congresswoman Alexandria Ocasio-Cortez, who has endorsed the politician whose presidential campaign she worked on in 2016. Coming soon after Sanders’ campaign wobbled after revealing he’d suffered a heart attack (it appears to have slowed him down very little), the backing from a prominent “progressive” (to use a term favoured in the US) was a huge boon – especially against the threat of Elizabeth Warren, a rival nearby on the left calling for “big, structural change”.

Sanders can also lay claim to fanatical online support, led by members of the Chapo Traphouse podcast, which is at the vanguard of the “dirtbag left” that is damning of “liberals” offering less than full-throated support for big change.

Their backing speaks to Sanders’ appeal outside the traditional structures that prop up campaigns. Big money donors have been shunned and instead Sanders relies on an expansive volunteer network that has helped make him a fundraising powerhouse. He has received more than 5m donations since entering the race, which is more “than any campaign has received at this point in a presidential election in the history of our country”. 

More controversially, Sanders has been endorsed by the podcaster Joe Rogan, one of the most influential figures in the “new” US media. While commanding millions of listeners, Rogan has drawn criticism for giving a platform to conspiracy theories and airing alleged racist and transphobic views – meaning his support for Sanders has sat uncomfortably with both liberals and progressives.

The British connection

New York-born Sanders has an older brother, Larry, who has been a politician in the UK as a Green Party county councillor in Oxfordshire. Larry has stood unsuccessfully as a Westminster candidate several times, including an attempt to replace David Cameron in the ex-prime minister’s old constituency in 2016.

Speaking of family, Curb Your Enthusiasm star Larry David has impersonated  Sanders since 2015 when appearing on the comedy show Saturday Night Live. David has said the role is not much of a stretch since they are both balding, somewhat grouchy older Jewish men from Brooklyn. What’s more, a 2017 genealogy TV show revealed the pair are distantly related.