Both are unlikely political sensations who were long consigned to the fringes: Bernie Sanders, an octogenarian US senator who inspired an army of voters far younger than himself; and Mick Lynch, a former blacklisted construction worker and child of Irish immigrants who, as the leader of the Rail, Maritime and Transport Workers union (RMT), shot to national prominence when he humbled hostile but underinformed broadcast journalists. “I think Lynch is touching a nerve,” Sanders says.

The de facto leader of the US left has swung his considerable political heft behind a new campaign – Enough Is Enough – launched to fight Britain’s mounting cost of living crisis, which was founded in part by Lynch and the RMT. It has certainly touched a nerve: at a recent rally in Clapham, south London, many of those who had queued around the block were turned away for lack of space. “‘Enough is enough’, funnily enough, is an expression we use a lot here,” Sanders says. “People are sick and tired of often working longer hours for low wages; sick and tired of their kids having a lower standard of living than them; and they’re sick and tired of billionaires getting richer and richer while they fall behind.

“Why, with all this new tech out there, are they not seeing an improved standard of living? Why not more equality, rather than less equality? Why are living standards deteriorating, not improving? Lynch is asking that, Enough Is Enough is asking that – and it’s hitting a nerve, because people are tired of being ignored while the rich get richer.”

Political cut-through is something Sanders knows a lot about, but it was only something he really achieved in his 70s. Born into a working-class Jewish family in New York, he became the mayor of Burlington, Vermont, at 40, later becoming a House representative and a senator. A longstanding independent, albeit one who has frequently allied with the Democratic party, Sanders championed causes long eschewed by mainstream Democrats, such as universal healthcare, the abolition of student fees, workers’ rights and the anti-war movement. But his dramatic rise – when he was transformed from a marginalised figure to a frontrunner for the Democratic presidential nomination in 2016 – was driven by two major factors.

One was the financial crash, which exposed inequalities and insecurities that disproportionately fell on the backs of younger Americans. The other was the expectations raised by the election of Barack Obama in 2008, which, for millions of Americans with stagnating living standards, ultimately felt dashed. Although neither his 2016 or 2020 bids succeeded, they mobilised a movement that revitalised the US left and transformed it into a major political force in the Democratic party and beyond.

This brings his attention back to a perennial passion – and what he wants to talk to me about: the prospects of the US labour movement. We speak over the phone, but he hits all his rousing lines with the zest of a platform rally. The thread that runs through all his answers is class politics. This is less of a novelty in progressive politics on the British side of the Atlantic – to rousing cheers at a recent Enough Is Enough rally, Lynch proclaimed: “The working class is back” – but it was long considered alien in a US that peddled a myth of classlessness. This was a politically convenient myth in a country where, Sanders notes, three rich men have more wealth than the poorest half.

But the Brooklyn-born Vermont senator has a new mission: to deploy his political weight behind efforts to unite the struggles of the US and British labour movements. On Wednesday, Sanders will bring his trademark oratory to an RMT rally in central London.

The labour movements in the US and Britain are significantly weaker than most of their western counterparts. In the US, trade unions had been long hobbled by “red scares” and anti-union so-called “right to work” laws, but they were severely weakened under Ronald Reagan, whose administration, in 1981, fired more than 11,000 striking air traffic controllers to send a salutory lesson to other workers. Today, little more than one in 10 US workers are unionised. British trade unionism did not suffer such a comprehensive rout, but the number of organised workers – about a quarter of the workforce – is half the level of the peak in 1979.

Does Sanders believe both labour movements are learning lessons? “I think what we’re beginning to see here in the US is a significant acceleration of trade union organising,” he says. “We are seeing more workers organising in unions, filing with the National Labour Relations Board [NLRB] to get certification – more than for a very long time.”

What has made him particularly optimistic is workers’ struggles in the union deserts of Starbucks and Amazon. Sanders recently joined striking Starbucks workers on a picket line in Boston. After more than 85 union organisers were fired by the coffee chain in recent months – the NLRB has filed multiple complaints against the firm – his support has boosted the national profile of the fight. “In Starbucks and Amazon, hundreds are joining unions – in Amazon, they’re taking on Jeff Bezos, the second wealthiest person in the world. We’re seeing struggles in university campuses, hospitals, nurses – we’re seeing unprecedented organising compared to what we’ve seen in recent years.”

But he touches on an apparent contradiction: “While the middle classes decline while the rich become richer and richer, there’s more support for the trade union movement in the US – people feel much stronger about unions than previously.” And he is right: last year, 68% of Americans told pollsters they approved of unions, the highest level since 1965, while polling in the UK has shown that most working-age Britons back the current wave of strikes. Yet that hasn’t translated into most joining a union. Why?

“In the US, corporations make it very hard for workers to exercise their constitutional rights to form a union,” Sanders says. “[Last Wednesday], the NLRB found Starbucks had fired workers and rescheduled those shop workers who were forming unions – which is illegal. We are seeing companies threatening workers that they’ll go to China. There’s massive corporate opposition to workers forming unions in the country.”

He highlights another formidable barrier: “We’ve got a media in the country which is certainly not sympathetic to unions, which will very rarely discuss the benefits of unions, like better working conditions, wages, pensions, et cetera, et cetera. The media is obviously owned by a handful of large corporations who don’t talk about class issues, economic issues. All of that contributes to making it harder for workers to become organised.”

But there is a tradition of militancy among US workers, despite attempts to scrub it out, not least in the 1950s under McCarthyism. During the Great Depression of the 1930s, waves of strikes rippled across US society. Does Sanders see a parallel? “Yes, I do. In the 1930s, there was a massive increase in organising and membership, and workers fought valiantly – they did sit-ins, took on powerful interests. What we are seeing now is real frustration in terms of inflation accounting for wages, with the average US worker earning less than almost 50 years ago – taking into account productivity gains, slightly worse than then. That’s insane!”

Given the likes of Starbucks have so long succeeded in suppressing labour organising, why has there been a blaze of activity? “I’ll tell you why, in my view: a lot of Starbucks workers are younger people. Many of them have college degrees and they’re looking around them: their wages aren’t keeping up with inflation, they can’t afford housing or healthcare or student debt, they’re falling further and further behind compared to their parents, and they’re standing up to the owner of Starbucks – Howard Schultz – saying: ‘You’re worth $4bn! What’s your problem with allowing us to organise workers?’ And his response is simply to try and fire workers and intimidate them. To some degree, this is a multiracial generational fight – primarily of younger people, but not exclusively – standing up to a billionaire.”

Starbucks has denied all allegations of retaliation. A spokesperson told the Guardian previously that “these individuals are no longer with Starbucks for store policy violations. A partner’s interest in a union does not exempt them from the standards we have always held. We will continue enforcing our policies consistently for all partners.”

But the relationship between the US labour movement and younger progressives has not always been harmonious, to say the least. In the 1960s and 1970s, US labour was led by the gruff former plumber George Meany, a zealous supporter of the Vietnam war, who relished denouncing student protesters as “kookies”. The nadir came in the form of the so-called Hard Hat Riot of 1970, when hundreds of construction and office workers physically attacked student protesters in New York. Is there hope this time for solidarity between trade unionists and the rising US younger left?

“That kind of unity is something we are working on very hard,” says Sanders. “I’ve now held three rallies with progressive union leaderships – with Sean O’Brien, the new president of the Teamsters, and Sarah Nelson, the president of the Association of Flight Attendants – in Chicago, Philadelphia and Boston. What we see at these rallies is unionists coming together with younger progressives – and the unity of those forces, young people fighting for economic and racial justice with a union movement, has incredible potential. To answer the question: it’s absolutely imperative we bring them together – and we are trying to do that.”

When US workers fought bosses in the 1930s, they enjoyed the advantage of the sympathy and political muscle of the president, Franklin D Roosevelt. Joe Biden has repeatedly vowed to be “the most pro-union president ever”, but his career has long been wedded to establishment and “centrist” factions in the Democratic party. Sanders says he knows the president “reasonably well” and points to the 110-page policy platform his team hammered out with Biden’s campaign team in 2020, with taskforces covering areas ranging from healthcare to the environment.

“What the president recognised is that there was, and is, a movement of working people, of young people, who are sick and tired of the status quo, and I think, when we did the American Rescue Plan [to help the US through the pandemic], it was one of the most consequential pieces of legislation for working people in modern history. When we did the Build Back Better legislation [a huge package of measures related to social policy and the climate crisis], it had the support of the president for a multibillion-dollar transformational programme, and it was sabotaged by a couple of conservative senators, but he said: ‘I will stand by the working people of the country and take on the big monied interests.’”

This differs from some of the more pessimistic narratives about Biden from the US left, which is still reeling from Sanders’ two presidential-nomination defeats. But his optimism springs not so much from naivety about Biden as from a firm belief in the ability of struggles from below to bend the powerful to act in workers’ interests. “You’re seeing a progressive movement of people in every state of this country which is beginning to go beyond incremental politics, asking: ‘How does it happen that every rich country on Earth – including the UK – has universal healthcare, while we have a dysfunctional system? Why in other countries is university education free, when in this country it’s outrageously expensive?”

I put it to him that his campaigns tapped into discontent, but magnified it and gave it direction. “What my campaign did was to raise issues, and the establishment suddenly discovered millions of people not happy with the status quo who wanted transformational change,” he says. Sanders gives the example of the president last week committing to cancel up to $10,000 (£8,500) of student debt. “Did it go as far as I wanted? No, but is it a significant step forward to alleviate the terrible burden that young people are suffering? Yes, it will help a lot.”

Another example is the recently passed Inflation Reduction Act, which, among other things, lowers prescription drug prices and promotes clean energy. “Again, it didn’t go as far as we campaigned on, but, on many of those issues, part of what we demanded has been implemented.”

What next for the US left? The youthful optimism of this 80-year-old senator appears limitless. Next, he says, they will grow the labour movement and tie it to the progressive movement. “You may or may not know, but, come January, in terms of politics, there will be a stronger underlying progressive presence in the House than at any time in modern history. We are seeing accomplishments at the political level, at the organising level, so we are making progress.”

Yet all of this relies on forcing a president to go beyond his comfort zone. Sanders remains one of the most popular politicians in the US, and his campaigns encouraged a galvanised US left to dare to dream of achieving outright political power. What lessons would a future campaign learn from his attempts, which transformed political debate in the US, but failed to secure him the presidency?

Sanders does a laugh anyone will recognise – the “I do not want to talk about this now” laugh. “That’s a long question – a very long question!” Again, he highlights his campaigns’ signature accomplishments – underlining that “a significant part of society is not happy with the status quo, that they’re sick and tired of income and wealth inequality and they want fundamental changes in our economic and political system”. But he clearly believes he was hobbled by establishment hostility. “When you take on the political establishment and the media establishment and the corporate establishment … it’s not an easy thing to do. We need time and we certainly didn’t have that luxury.”

I wonder, too, if he recognises that Enough Is Enough has emerged in large part because of a vacuum left by a Labour leadership that has abandoned any pretence of transformative change. Sanders is diplomatic. “I think it’s not dissimilar to what we’re seeing in the Democratic party here – I’m not commenting on the Labour party; I don’t know enough,” he says. Referring to traditional left-of-centre parties struggling in the global north, he adds: “Because working-class people are increasingly alienated from the political process, those parties are not delivering for them. That’s why the Democrats have a choice to make: are they the party of the working class or the elite?”

The legacy of Sanders, surely, is that he brought together otherwise fragmented and disillusioned pockets of discontent into a highly visible and articulate movement with confident demands. Maybe – just maybe – he can help pull off the same trick by helping to unite the increasingly assertive labour movements on both sides of the Atlantic.