On the left lapel of Jean-Luc Mélenchon's suit jacket is a small red badge; a triangle shape. He wears it, he says, to remember les deportés - leftists and other political undesirables who were persecuted during the Nazi occupation of France “A salaud of a journalist illustrated an article with a drawing which suggested that there was no difference between me and [the far-right politician] Marine Le Pen. So I wore this badge, to say there are thousands of dead - that's the difference.” But the triangle’s three sides, says Mélenchon, when we meet in the grand former booking hall of St Pancras station - now a posh restaurant - also represent a historic demand of the workers’ movement: eight hours work, eight hours rest and eight hours leisure.

It's partly through symbolic gestures like these - a mix of history, sentiment and a confrontational stance – that Jean-Luc Mélenchon has been able to breathe new life into a French left that, by his own admission, was “40 seconds from death”. At the start of this year, Mélenchon, a former minister in the Socialist government of Lionel Jospin, in the early 2000s, shot to international attention as an unexpected challenger in the French presidential election. Until that point, the campaign had been dominated by the presence of Le Pen, the new, supposedly more voter-friendly leader of the Front National. Despite dire economic circumstances, her racist agenda had dominated the campaign, forcing the two main challengers, Francois Hollande and Nicolas Sarkozy, to try and out-tough one another over immigration, or Islam in France. (During a televised presidential debate the pair spent an alarming amount of time arguing over whether giving non-EU citizens the right to vote in local elections would lead to “women-only swimming sessions”, and with it, the collapse of French values.) But Mélenchon, candidate for the Front de Gauche (the “left front”), a coalition of disaffected socialists, communists and environmentalists formed in 2009, had decided to confront Le Pen head on, opposing her party programme “line by line” with an unashamedly left-wing manifesto, dismissing her memorably during an on air exchange as a “semi-demented bat” and making it his stated aim to humiliate her at the ballot box.

Mélenchon failed to achieve his immediate goal - he was beaten into fourth place by Le Pen – but his own 11 per cent share of the vote represented a wider shift to the left, one that saw the downfall of president Sarkozy. When we meet, the day after he has delivered a lecture to a packed-out audience at University College London, he tells me that his role has become all the more necessary since the election. The centre-left Hollande’s honeymoon period is firmly over (opinion polls reveal widespread public scepticism that he will be able to solve unemployment, the public deficit, or the decline of French industry), and the media has recast Le Pen in the role of “seductive devil”.

“Each time there is a problem,” he tells me, “the media say ‘uh-oh, this is going to benefit Madame Le Pen’. It's a way of making people accept the current situation – either you accept this, or you accept the extreme right. Stay at home if you are angry, otherwise you will do harm.” Mélenchon aims to break that deadlock. “Democracy is not consensus. It's a mode of regulating conflict and difference. And to deny conflict in society is as dangerous as it is to mental health to deny the conflicts that we experience as human beings.”

Listening to Mélenchon speak, it is clear that there is more to his politics than the crude rabble-rousing it is often caricatured as. “He likes to talk,” one of his aides warned me as I arrived at St Pancras; nothing unusual for a politician, you might think - but a question about press bias, for example, kicks off a 15-minute discourse, delivered in booming voice, with an eye for the poetic image, on relations of production in the media industry, and the need to end “precarity” of employment among journalists and give them the resources to “ensure production of dignified intellectual quality”. These are radical ideas in the truest sense of the word – a problem analysed and addressed at its roots.

Yet he's a politician, nonetheless. One criticism of the French left is that its rigid commitment to Republican secularism (laicité) has left it ill-equipped to deal with islamophobia. When a rival left-wing party, for instance, stood a female Muslim candidate at local elections in 2010, Mélenchon joined the chorus of critical voices, on the grounds that her headscarf was a violation of laicité. I ask if this approach can end up excluding Muslim women from political life, but he responds by talking about the burka - eloquently, persuasively, but not really the point.

I press him on this. “Either human rights are universal or they are not,” he responds. “There is a common human interest, and we can only discover that interest by putting aside the particular revealed truths in which we believe. That doesn't mean we need to abandon those beliefs.”

That common interest, in Mélenchon’s eyes, is the environment. (“There is only one ecosystem compatible with human life. In that, we are all equals.”) And what makes the Front de Gauche different, Mélenchon says, is that it has been designed to accommodate differences of opinion - a process he illustrates by way of overlapping napkins on the restaurant table. In this, it has much in common with the emerging anti-austerity movements in southern Europe - such Greece's Syriza - which have rejected conventional politics for a broad-based grassroots approach. Only in this case, it’s happening in one of the eurozone's largest economies. “We are the second power on the continent,” Mélenchon says. “We are soon going to have the largest population. Europe can't be built without us, which means it can't be built against us. Germany can’t expect the rest of Europe to follow policies aimed at old people who want capitalism to pay for their retirement. We are a young people, we need schools, we need health care, we need work.”

“This is not to ask for things only for the French,” he continues. “For me, French nationality is not ethnic, it’s a political project: liberty, equality, fraternity. And we don't ask these things only for the French. You have the same interests, no?”

If his rhetoric is lofty, then so are his political ambitions: Mélenchon declared in a recent interview with French media that he was “ready” to become Francois Hollande’s prime minister. This wasn’t a request, he tells me, but to project an air of “confidence” that will “inspire my compatriots”. He continues: “For me it's a way of saying to Hollande, that because soon you're going to be facing the dirt, you should know that there exists another alternative, to the left.”