One thing is immediately striking: as he addresses primary rallies across America, arms flailing like a giant bird coming in to land, snow-white hair fizzing skywards like Doc Brown’s in Back to the Future, eyes startled behind the robust spectacles he has worn since childhood, Bernie Sanders looks quite unlike any other presidential candidate.

Perhaps the surprise in those eyes is sparked by the size of the crowds Sanders has been attracting. They are enormous, rivalling the numbers who turned out for Barack Obama back in 2008, and unprecedented for a candidate who is not shy of describing himself as a socialist: 28,000 in Portland and LA, 25,000 in Boston and 15,000 in Seattle. Even in Dallas, not a renowned centre of radicalism, 8,000 turned out to “feel the Bern”.

In these days when slick suits and expensive haircuts are increasingly a turn-off for a public weary of smooth politicians they see as delivering only for the wealthy, Sanders’s persona, like that of Jeremy Corbyn, his equally unkempt British counterpart, has proved popular. But it is his message – an angry chronicling of the depredations facing so many Americans and a solid social-democratic programme for putting things right – that is really pulling in the crowds. Sanders, who is 74, and the main challenger to Hillary Clinton for the Democratic nomination, doesn’t just look different. With his confident calls for a “revolution” to break up the banks and impose higher taxes on the rich, he doesn’t sound like any other recent presidential contender, either.

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I first met Bernie Sanders in 1996. I was the publisher of his book Outsider in the House, a political autobiography that appeared the following year (and which has just been reissued by Verso with a new foreword, and more than a hint of optimism, as Outsider in the White House). The occasion was a benefit concert during his successful bid to be re-elected to the House of Representatives from the small, rural state of Vermont.

Sanders’s early years are not well documented, least of all by him. He devotes less than three of the 300 pages in Outsider to the first three decades of his life. He doesn’t much care for the “humble roots” narrative beloved of so many politicians, generally millionaires whose ancestors lived in broken-down cabins. But the raw material is certainly there. The son of Polish immigrants, Sanders grew up in a working-class Jewish family in Flatbush, Brooklyn. At home, money was tight: “Every major household purchase . . . would be accompanied by a fight between my parents as to whether we could afford it,” he wrote.

It was an achievement to gain admission to the University of Chicago, and though he described himself as “not a good student”, that was a result of sacrificing coursework to the cause of social activism. He settled permanently in Vermont at the age of 27, having bought an 85-acre farm in the north of the state for $2,500. Four years later he moved to Burlington, the state capital, where he became involved in city politics, at first in the tiny Liberty Union Party and then as an independent. In 1981 he was inaugurated as mayor and commenced a series of tilts at the state’s congressional seat. He finally entered the House of Representatives in 1991 – the first independent candidate to enter Congress in 40 years.

By the time I encountered him, Sanders was seeking to defend his seat for the third time. The concert where we met was taking place in an old art-deco theatre in Brattleboro, perhaps the most hippiefied community in a state where tie-dye remains as ubiquitous as dairy herds. It was headlined by Pete Seeger, who ran through a panoply from his folk songbook to a packed crowd that knew all the words.

Ten years earlier, Mayor Sanders, a long-time admirer of Seeger, had recorded one of his songs, “Where Have All the Flowers Gone”, on a surreal folk/rap album. Now, he waited until Seeger had finished his set before taking the stage and, speaking in the only manner he seems to know – a gruff, shouted staccato – exhorted Vermonters to join him in the fight against Newt Gingrich and Bob Dole to stop the Republicans from taking over the presidency and the Senate. The response was rapturous. Sanders left the stage like a president concluding a State of the Union speech, gladhanding lines of admirers as he made his way out of the hall.

A few weeks later I met him again, this time at his congressional office in Washington, DC. On the wall of his office I spotted a plaque of Eugene Debs, who ran for Congress and the presidency on a socialist ticket, travelling to every part of the country on a train he called the Red Special and picking up 6 per cent of the popular vote in 1912, when he finished fourth, behind Woodrow Wilson, Theodore Roosevelt and William Howard Taft.

Sanders had invited me to lunch and so we headed off through the underground passageway that leads from the office building to the congressional dining room. We were accompanied along the way by one of his assistants who, in true West Wing style, peppered him with questions and made notes on a clipboard as we walked. We had just started our food when John Kasich, then congressman for Ohio and now governor of the state and a contender for the Republican presidential nomination, wandered over for a chat. Despite Kasich’s reputation as a fiscal conservative, it was evident that he and Sanders had a cordial relationship, and indeed, Sanders invited him to join us for lunch.

It was difficult to reconcile these two contrasting snapshots of Sanders: the rousing air punch in Vermont and the bridge-building handshake in DC. But the more one looks into his career, the clearer it becomes that this dual approach is integral to his remarkable political ascent. Sanders plays it quite differently inside and out, but he plays both sides very hard.

“Bernie doesn’t see a contradiction between working within the system and campaigning to change it,” the journalist Matt Taibbi told me, recalling the time when he shadowed Sanders for several weeks in 2005 while researching a piece for Rolling Stone. “I remember one Thursday afternoon I made a snarky comment about members of the House already sneaking off home for a long weekend and how it seemed to me that many of them were pretty lazy. Bernie scolded me, insisting that most of the people in Congress work very conscientiously. He doesn’t believe the system functions for ordinary people, but he’s not cynical about it either.”

This point was reiterated by Heather Gautney, an associate professor of sociology at Fordham University in New York who previously worked as a researcher in Sanders’s Senate office. “Working with Bernie in DC, I realised what a difficult place it was for someone more interested in movement-building than passing legislation,” Gautney said. “But Bernie was known for getting substantial chunks of the Republican vote in Vermont and he used that same skill to connect with some pretty unlikely allies in Congress.”

Sanders’s legislative record is strikingly good. In the decade after the Republicans took over the House of Representatives in 1995 no other lawmaker attached more amendments to bills that were voted on. He achieved this by using his position as an independent to put together coalitions that spanned both of the main parties, and also by sheer hard work. In his Rolling Stone article, Taibbi describes Sanders waiting patiently for hours to table an amendment in the office of the House rules committee, “a tiny, airless closet deep in the labyrinth of the Capitol where some of the very meanest people on Earth spend their days cleaning democracy like a fish”.

Sanders’s method of working across party lines is not without its critics. Especially on the left, there are voices that wonder if the compromises that inevitably accompany playing the system in DC are too large. Many of Sanders’s positions on foreign policy have skewed towards the militarism and careless disregard for human rights that prevail among the Washington establishment. Although notably, and unlike Hillary Clinton, he opposed the initial vote on the Iraq War, Sanders voted for every bill that came before Congress to fund the occupations of Afghanistan and Iraq. He has supported basing the new F-35 fighter plane at Burlington Airport in Vermont, despite widespread concern from residents about the environmental impact. And he did not oppose the Senate resolution that supported Israel’s attack on Gaza in 2014, which left as many as 2,200 Palestinians dead.

Sanders is clearly happier talking about problems inside the US than foreign policy. In his opening statement to last Saturday’s televised debate between the Democratic candidates, he segued awkwardly from condemning the attacks in Paris to excoriating America’s “rigged economy”. Yet on domestic issues, too, some of his stands have given progressives pause for thought: his opposition to the Trans-Pacific Partnership, a 12-country trade agreement championed by Barack Obama, has always been grounded in an argument in favour of saving jobs for American workers, rather than any notion of international solidarity. His slowness in expressing support for the burgeoning Black Lives Matter movement, something which his campaign has latterly striven hard to correct, was less of a surprise to those aware of his consistent support for the police union while mayor of Burlington. And his position on guns (he voted against the Brady Bill, which mandated background checks on buyers of firearms) is the only area in which Clinton outflanks him to the left.

But perhaps the biggest issue for many progressives is Sanders’s decision to run for president through, rather than outside, the Democratic primary. Though he began his political career in the Liberty Union Party and has stood in every election since as an independent, he is, as Howard Dean, the progressives’ challenger in the Democratic primary of 2003/2004, put it, “basically . . . a Democrat . . . [who] votes with the Democrats 98 per cent of the time”.

As Sanders relates in Outsider in the House, faced in 1996 with the choice of backing Ralph Nader, “a personal friend and an exemplary progressive” running as an independent, or Bill Clinton, whose policies on health care, welfare reform, trade, gay marriage and military spending he sharply disagreed with, Sanders decided to “support” Clinton. “Perhaps ‘support’ is too strong a word,” he frets in the book. “I’m planning no press conferences to push his candidacy, and will do no campaigning for him. I will vote for him, and make that public.”

Sanders has called for a vote for the Democratic nominee in every presidential election since Jimmy Carter left office in 1981, and early this month, on ABC’s This Week, he appeared to have completed a long transition, asserting: “I am a Democrat now.”

This failure to build an electoral force outside the Democrats always leads to a dead end, according to Anthony Arnove, a prominent member of the International Socialist Organisation (ISO) who is also a publisher and literary agent representing a range of leftish writers, including Arundhati Roy. “We’ve seen it over and over,” Arnove said: “a left challenge fires up the base and is then defeated in the primaries by a centrist, or, more accurately, right-wing candidate, who goes on to betray everything those people were mobilised around.”

Sanders’s fundraising almost matched Clinton’s over the summer – in the third quarter they raised $26m and $28m, respectively – and in September he became the first candidate to attract more than a million individual donations. (The average donation to his campaign has been $30.) But his dip in the polls after Hillary’s strong performances in the first nationally televised primary debate, and then again at her House select committee hearing on the 2012 attack on the US consulate in Benghazi, Libya, suggests he will struggle to win the nomination. As of early November he trailed Clinton nationally by 25 points.

In Arnove’s view, Sanders “won’t get further than Super Tuesday [at the beginning of March], when he’ll direct his base to vote for Clinton. This is exactly how the Democrats become a graveyard for progressive politics, when what we desperately need are social movements that can remain independent of both establishment parties and push for their own agenda.”

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The revolution to which Sanders often refers is a long way from the sort envisaged by Arnove’s ISO. He is pursuing a fairer capitalism, not its overthrow. “He’s not Trotsky,” as Taibbi put it to me. But there are those inside his campaign who think its primary focus should be building a grass-roots organisation capable of transcending the four-yearly coming together of presidential elections, to create a more permanent basis for a broad, progressive movement.

One such advocate is Adolph Reed, a writer and academic who is campaigning for Sanders in South Carolina. Working with local unions and Labor for Bernie Sanders 2016, which has 70,000 signed-up members, Reed sees the potential in using Sanders’s programme, with its emphasis on basic economic demands such as the minimum wage, universal health care and free college tuition, as a way of drawing together various groups campaigning around single issues such as housing and police racism.

For Reed, who is black, class trumps race as the key to building a movement. “In New Orleans everyone talked about Katrina as having a devastating effect on black people in the city, which of course it did. But when you analyse it, class was a much better predictor of who suffered most there,” he told me. The centre of a class-based movement, Reed argues, will have to be provided by the trade unions. “Despite the fashionability of protests without any specific demands or elected leaderships, no movement initiative is going to have staying power without being anchored in the trade unions.”

Recruiting the unions to work alongside Sanders’s campaign in the way Reed envisages isn’t easy. The American Federation of Teachers and the machinists’ union have already thrown in their lot with Hillary Clinton. And Richard Trumka, the president of the AFL-CIO (America’s national federation of trade unions), has warned individual unions against coming out for Sanders. But Reed can point to significant declarations of support, from postal workers and the National Nurses Union. The AFL-CIO chapters in Vermont and, more surprisingly, South Carolina have also backed his run.

“It’s important to keep Bernie in the race for as long as possible, but the ultimate objective is to develop structures that can continue beyond the election,” Reed said. “It’s premature to say what this network will look like, but Bernie’s campaign provides an important boost to putting it in place.”

***

From Jesse Jackson to Dennis Kuci­nich to Howard Dean, an array of people’s champions has made a splash in the recent history of Democratic presidential primaries. None, however, has been as explicitly critical of capitalism (or so gruff about it) as Bernie Sanders. His no-nonsense, class-based politics are a measure of how the disenchantment with the ideology of a free market that arrived like a train in the 1980s and ran off the rails in 2008 is now finding its way into the mainstream.

Up until now, the critical moments of left advance in America – the Seattle WTO protests, the anti-war movement, Occupy Wall Street, the campaign for gay rights and, today, Black Lives Matter – have occurred outside electoral politics. There are a couple of good reasons for this. The US electoral system, like Britain’s, makes third-party challenges extraordinarily difficult. And inside the Democratic Party these movements would have been crushed by a conservative leadership around the Democratic National Committee, put in place by Bill Clinton.

One result is a paucity of new progressive voices inside the party. At a moment when, as Gramsci once put it, the old order no longer works but the new order has not yet been born, Sanders, with his New Deal politics and firebrand demeanour, seems not so much a successor to the old order as a throwback to a time that pre-dates it, when politicians spoke with conviction and the society they represented was less unfair. As such, he provides a staging post for a new progressive consciousness (according to a poll by Pew at the end of 2011, more Americans aged 18 to 29 would prefer to live under socialism than under capitalism) that is not yet sufficiently coherent to enter mainstream politics in its own right, either through a serious third-party challenge or the transformation of the Democratic Party.

As a middle-class white man, Sanders has been able to get a pass to promote bold positions that someone with a less privileged identity might have found hard to sell. And his age, paradoxically, has proved not to be a disadvantage with a youthful constituency dismayed by the surrender to expedience that disfigures so much of contemporary American politics. His record has been constant over such a long period that, again like Jeremy Corbyn, he can be relied on not to sell out. Though his politics are less radical, his venerability provides a messianic cloak from the same closet as the one worn by Noam Chomsky, another hero for many young progressives.

So it’s not just today’s professionally polished politicians to whom Sanders presents a stark contrast. Recent progressive movements have embraced an identity politics that was much less prevalent when Sanders started out back in 1970s Vermont. In order to forge the sorts of alliances that are necessary to mount a credible challenge on the national political stage, they will likely have to borrow extensively from his unifying class politics. But their leadership will be younger, blacker, less straight and less masculine than Sanders. In that sense, he represents the last hurrah for the old white guy.

Colin Robinson is co-publisher at OR Books (orbooks.com), based in New York