Up until last year, the Sanders brothers were a political curiosity: Bernie in Washington, the longest-serving independent in congressional history, and Larry with a role in the Green party over here (he became health spokesman last year, having joined the Greens at the turn of the century).

Now, “it’s very odd for me”, Larry says equably, sitting in a tiny, messy office space rented for his byelection campaign in Witney, the seat – you will recall – left vacant by the resignation of the worst prime minister since the second world war. “After so many years, I’ve suddenly become a celebrity. And all these young girls want to have pictures taken with me.”

As I arrived, it was actually a young man who had approached him, offering to give out leaflets and promising his vote, and that of his girlfriend. Sanders was unhurried and appreciative, listening, taking his details, assuring him that there was plenty to do. I haven’t met a sharper, fitter 82-year-old since I interviewed an astronaut (Charlie Duke, thanks for asking), but Sanders nevertheless carries the air of a different generation’s politics; attentiveness, pens and scraps of paper, plain answers, rosettes, unembarrassed morality, a wonderfully long view.

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Talking about New Labour at one point – his first involvement in British politics was with the Labour, until Blair’s famous promise to maintain Tory spending cuts – he says: “The nebulous supporters of New Labour, I don’t know who they are. It might be like the Vietnam war: some people were violently against it, some people were violently against the people who were against it, but I never met anybody who was for it.”

The rise of Donald Trump doesn’t surprise him, because he remembers McCarthyism, and “always knew there was this strange underbelly of American politics”. He recalls clashing with longshoremen in New York in the 50s, and then, 40 years later, battling Labour councillors over cuts to adult social care in Oxford.

Social justice has been his life’s work; his actual career he describes impartially and rather distantly, like some DIY he did once in a house long sold. “It’s probably the most successful paid work I’ve ever done,” he remarks of his first job in England, as a community worker. “Fortunately, nobody knew what a community worker was supposed to do. I decided what I was supposed to do was bring people together who needed things from each other, or who needed help to campaign.”

He has none of that purist glorification of folk politics, though, where activism has to happen one street at a time in order to have meaning. “There are lots of ideas you can get only from community politics, but if you want to change things, you have to play the game. Bernard’s example is that the big, standup stuff, the big speech with the big stadium, is very powerful. With mass media, you can build a tremendous amount.”

Facebook Twitter Pinterest Campaigning in Witney in the byelection campaign. Photograph: Carl Court/Getty Images

Later, chuckling, he remembers seeing Bernie in Vermont in the 60s, saying broadly what he’s saying now. “Then, he would be doing it to an audience of 20 people. To see him in front of 25,000 people, saying things that, until the day before yesterday, were implausible ...”

The conversation often returns, naturally, to his younger brother, for whom he has great admiration. He ascribes their bond partly to the early death of their parents. “I was about 25 and Bernard was about 19 when our mother died. We were orphans by the time I was 27. So I think we felt very close.” It is doubtless also down to the fact that their politics are “pretty nearly indivisible – 99.9%. Our parents were very similar. They would not have known a 10th of what I know, or a 100th of what Bernard knows. They just thought that Franklin Roosevelt and the New Deal was the way to run a country. They had no doubts whatsoever. They weren’t following the ins and outs. They thought that a government ought to be working for its people. They couldn’t imagine why anybody would think otherwise.”

Larry Sanders came to Britain in the late 60s – his opinion varies as to whether it was 68 or 69 – following a woman who would become his wife. He was in his early 30s, and had already spent a few politically active years in New York, fighting urban renewal, which he called “poor people removal. Which it was ... One of the reasons I was easy to convince to come to England was that, in New York, everything irritated me. I’d walk down the street and think, ‘Why is that there? Why has that happened?’ Whereas for a few years after I came here, I thought, this is just a street.”

He had done two years at Harvard law school, but dropped out while his mother was dying; then he couldn’t return after her death because his father became depressed, then ill himself. Over here, he did a masters in social work at Oxford, and eventually – when the relaxing impartiality wore off – joined the Labour party, which initially refused him on the grounds that he was a Trotskyist (which he never was, although he says it wasn’t a wild accusation: “There was quite a strong, numerically very small, but strong Trotskyist movement. There was WSL [Workers’ Socialist League] – people used to call them the weasels – and I think a predecessor ... you’re too young and I’m too old to remember this stuff, but there were plenty of them about”).

While he was teaching social work in London and living in Oxford, in the 70s, his county council decided to close down all the nurseries (they were “childminding on the rates” the Tory candidate said), and this was Sanders’ first real entry into British politics, occupying – which functionally meant working in – one of the nurseries until the bailiffs came. “And that’s the hardest work I’ve ever done in my life. Those children are so fierce. There was one little boy whose goal in life was to kick me in the balls. No matter how I manoeuvred, that kid was so flexible.”

He is – I say this without exaggeration – the first politician I have ever heard acknowledge that childcare is not just sentimentally but literally harder than all other jobs. The campaign was a success – they saved 80% of the provision – but it left him with no illusions about leftwing politics. “Afterwards, we had a disastrous time, when the mothers of the children and the politicos who had moved into the campaign had a knockdown drag out.” Talking to him is a bit like watching Marlon Brando: I get the gist, but subtitles would help. “The campaign split,” he explains.

Facebook Twitter Pinterest With Bernie. Photograph: Larry Sanders/PA

Life intervened: tragically, his wife died when his daughter was 10 and his son, 14. When the kids went to university, he decided to go back to the US, to keep Bernard company – “He was very isolated in Washington, and Jane [his wife] was in Burlington most of the time.” He was in his 50s by this time, and there is something touching about the simplicity of his explanation – two adult brothers hanging out, complaining about each other’s snoring. Bernie made possibly the most direct intervention ever by such a senior US politician into UK politics, when he endorsed his brother in the Witney by-election last week. Larry had cast a tear-jerkingly moving vote for his brother on behalf of overseas Democrats at the party’s convention in Philadelphia. The way he tells his life chimes precisely with his politics – people before profits, adversity met with fellowship; it’s sad that it sounds exotic.

Anyway, through some abstruse tradition that, once you’ve been accepted in Harvard, they will move heaven and earth to make sure you graduate, he finished his law degree in the 90s and came back to the UK with the qualifications to lodge legal challenges against cuts to disability provision that the new Labour government had made necessary.

“You didn’t have to be terribly sophisticated to see what Blair was doing. To tell the truth, I didn’t get that bothered by Blair. Even in the 80s, members didn’t really think the leadership thought what they thought. But Blair was certainly destructive, the way Bill Clinton was destructive.” It was through the social justice, rather than climate change, that he came to the Green party, although environmentalism certainly grips him now. His main critique of Jeremy Corbyn is that “his own specific knowledge seems thin. He does say many of the right things about climate change and I trust him more than many politicians. But he doesn’t know a lot about it.”

Sanders’s chances of electoral success in Witney seem remote, in so far as anything can be described as likely or unlikely in our current politics. Except for about five minutes after Shaun Woodward crossed the floor in the 90s, the constituency has been solidly Conservative for as long as anybody can remember.

Facebook Twitter Pinterest Larry, left, and Bernie as children. Photograph: Larry Sanders/PA

Yet that doesn’t mean his views find no traction here. Last week, there was a hustings at which the Tory candidate, Robert Courts, was booed for his claim that the NHS was being perfectly well-funded, and that all the problems had been caused by the junior doctors (“Whether they started off anti-Tory, or his stupid answers helped, I don’t know,” says Sanders). Courts is an unpopular candidate anyway, an unabashed Brexiter in a staunchly remain area. The Lib Dems clearly have high hopes (Nick Clegg was everywhere you looked, on the train, on the high street, in a charity shop, like Timothy Claypole).

It’s still unclear whether this will deliver any votes to Sanders, but that’s not really the point: “It’s not an expectation of winning or getting a particular percentage. It’s just an opportunity to say what’s happening.” He certainly doesn’t see himself as bringing an unpopular message to hostile Conservative territory. “There are some underlying things that are the same everywhere. I talked in one stretch of an hour to a dozen young people, and all I have to say is ‘housing’. All 12 of them were living with their parents – they were either in education or working. That’s not only where they were now – they didn’t see any prospect of being able to move out. How do you defend a politics in which the majority of your young people can’t look forward to a decent place to live? This is absolutely basic.” What does he say to old people? “Old people, I can strike up a conversation immediately about pavements. The county council has had 40% taken out of its budget. It means they have cut care, but they also can’t afford to deal with potholes.”

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His arguments are confident and coherent, but he’s not optimistic about the Labour party getting itself into shape in time for the next election, nor about the possibility of a progressive alliance coagulating into an electoral force. The most he thinks that could achieve is a groundswell of parliamentary support for proportional representation, and after that, things might change. “I think there’s going to be a lot of suffering, and suffering is not really good for people politically, any more than it is personally. Ukip is not the same as the National Front, but it is a drastic thing. Historically, when people are in bad financial shape and you offer them a chance to hate somebody, a significant proportion of people will take that.” Furthermore, “if the alternative is some sort of revolution, I don’t see it and I don’t want it”. However, he returns, finally, to his brother’s success. “Before Bernard did it, nobody except him and me and his wife would have predicted it could happen.” In the quagmire of the US presidential election, it’s hard to remember much that was hopeful. But there was, unmistakably, a resurgence of socialist ambition. If it could happen there, why should something 99.9% similar not happen here?