A piece of pseudo-wisdom sometimes trotted out by political activists in Britain holds that the two main parties – Labour and Conservative – are just the same. Of course, a closer look reveals some fairly stark differences. With the election last year of Jeremy Corbyn as leader of the Labour Party, the distance between them and the Tories, in policy terms, is greater today than at any time since the early 1980s. Even during the Tony Blair years – when he was hated by some on the left, who dismissed him as a 'Red Tory' – many of Labour's policies (record NHS funding, the minimum wage, tax credits) would have been inconceivable under Conservative rule.

Thus, when a person lazily drones on about politicians being the same, it is usually best just to shrug and walk away.

Until fairly recently, this rule would not have applied to the United States. Relative to the left-right divide over here, Democrat and Republic politicians have often resembled a single party sliced in two. Mainstream Democrats are as obsequious with Wall Street as their Republican counterparts. During the Bill Clinton era, income inequality soared. Meanwhile, investment taxes were cut for the rich while President Clinton simultaneously introduced draconian welfare reforms, boasting of putting an end to what he called "welfare as we know it".

In recent times, Donald Trump and Bernie Sanders have turned American politics into something more familiar to European audiences. In the race for the 2016 Presidency, civilised Brits have worried about the rise of business tycoon Donald Trump, and leftists have taken inspiration from the campaign of Vermont Senator Bernie Sanders. Trump leads the field of GOP candidates in the race for the presidency by more than 20 points; meanwhile, Sanders is breathing down the neck of the Democratic Party frontrunner, and consummate establishment figure, Hillary Clinton.

Trump represents a phenomenon we've already felt the murmurings of in Europe since 2008 – the angry rise of the 'little guy'. A billionaire himself, Trump is the small man writ large. His supporters are older, poorly educated and earn less than the average Republican. The profile of the average Trump supporter is strikingly similar to those attracted by Nigel Farage in the UK. Like Trump, Farage has tapped into the concerns of what the academics Robert Ford and Matthew Goodwin call the 'left behind' voters – working and lower-middle-class voters who feel they are being shafted by globalisation.

Yet rather than seeking to redress the economic imbalance that results in swathes of the population struggling to keep their heads above water, the Trumps and Farages of the world have done what right-wing demagogues excel at: channelling economic grievances into neurotic prejudice. The American middle class is shrinking at an alarming rate; however, Trump has nothing to say about that – as a billionaire, he benefits nicely from the status quo – thus he prefers instead to stir up xenophobia in a 'look over there' fashion, blaming society's ills on Muslims, Mexicans and a 'liberal elite'.

Yet Donald Trump's success contains lessons for the American left. And it contains lessons for the left in Britain, too.

The 2008 global financial crash was caused by an unrestrained, laissez-faire capitalism. Yet across Europe it is social democrats, rather than hawkish neo-liberals, who have struggled to make headway in the aftermath of the crisis. British politics provides a stark example of this paradox. Prior to 2008, David Cameron and his chancellor George Osborne were calling for less, not more, regulation of the banks that subsequently crashed – yet it is Cameron and Osborne who have benefited most handsomely at the ballot box in the post-crisis landscape.

Right-wing hubris may have caused the financial system to come crashing down, but it is the social democratic left that has found itself in the doldrums in the aftermath. Even in France, where the centre-left is in power, President François Hollande is flailing badly in the polls. And it is no mere coincidence that the failure of Hollande's Socialists to articulate a convincing alternative has coincided with a surge in the fortunes of the French far-right. The lack of a rational response to the crisis has opened the door to an irrational one – enter stage right the bigots of the National Front.

Much like their European social democratic counterparts, in their dealings with Wall Street, American Democrats have preached moderation when moderation has become untenable. A Democrat may be in the White House, yet the middle class has continued to atrophy on President Obama's watch. Hillary Clinton, the centre-left's favoured candidate in 2016, apes the language of Bernie Sanders in front of left-wing audiences, yet as she frequently tells her big-money donors, she comes from "the Clinton school of economics". "I think people are very excited about Hillary," as one Wall Street insider told Politico.

The acquiescence of the American centre-left in the failures of right-wing economics has hastened the gradual disintegration of the centre. For the first time in years, Europeans can easily comprehend the ups and downs of American politics. For better or worse, the lazy mantra that politicians are all the same, even if decidedly shaky to begin with, no longer applies in the United States. The dark side of this is expressed in the rise of Donald Trump; the flipside has been Bernie Sanders's heroic push back against obscene inequality, the billionaire class and the identikit Democrats who facilitate the existence of both.

Across Western democracies, the failure of social democrats to articulate a coherent response to the 2008 crash – notwithstanding the muffled articulation of a tepid version of cutthroat capitalism – has held the door ajar for demagogues like Donald Trump and Marine Le Pen. But it has also provided a fresh opportunity for the left. In the US, Bernie Sanders is providing a stirring example of how you grasp it – according to polls, Sanders would defeat a Trump candidacy in a general election by 13 percentage points.

This ought to encourage the British left. But there is also a lesson here for so-called Labour Party 'moderates': Jeremy Corbyn may not be the answer, but as the British economy continues to stumble anaemically out of the shadow of 2008, danger also lurks in a fraying and discredited status quo.