The left has finally got its act together – in France. Watch out, Macron and Le Pen | Cole Stangler

Few thought the day would ever come, but an earthquake is about to shake up French politics. Ahead of next month’s legislative elections, France’s famously fractious leftwing parties have decided to join forces, with Europe Ecology-the Greens (EELV), the Socialist party (PS) and the French Communist party (PCF) coalescing around Jean-Luc Mélenchon’s left populist La France Insoumise (LFI) and uniting around a common set of candidates.

In France’s semi-presidential system, the legislative elections often go overlooked, but they play a decisive role in determining how much of the president’s agenda can actually be enacted. Without a majority in the national assembly, the president is forced to share power with a rival prime minister and cabinet – a situation known as “cohabitation”. It’s happened three times before, but not since 2002, when legislative elections were moved to take place just after the presidential election.

And yet, cohabitation is a real possibility in today’s highly volatile landscape, with polls suggesting the new unity coalition – the Nouvelle union populaire écologiste et sociale (Nupes) – on track to win a significant share of seats. Even if it does fall short of obtaining an outright majority and forming a government of its own, a newly empowered left bloc in parliament could pose headaches for the president, Emmanuel Macron, as he embarks on his second term in the Elysée.

Reaching a deal wasn’t easy and tensions remain, but the unity pact has been fuelled by a mix of shared ideological commitments, a heavy dose of self-interest and a certain willingness to compromise. Before the presidential elections, each of the coalition members was gunning for a prime spot in the legislatives. But then came Mélenchon’s strong performance in the first round of the presidentials, nearly beating Marine Le Pen to enter the final round. Any lingering doubts over the French left’s centre of gravity were laid to rest.

While La France Insoumise won 22% of the first-round vote, candidates for the Greens, Communists and Socialists each earned less than 5% – lacklustre results that couldn’t be papered over and that also had the effect of depriving them of generous public subsidies. Voters had spoken and they preferred Mélenchon’s line: massive state spending to tackle the climate crisis; wealth redistribution and worker protections; an unapologetic response to rising racism and xenophobia; a willingness to bypass European Union rules if they prevent such policies from being carried out.

Shortly after the second round, La France Insoumise extended an invitation to negotiate on the basis of this programme – and the offer was taken up. While Mélenchon’s party remained in the driver’s seat, it also proved willing to make concessions: it will supply a majority of the coalition’s candidates, but the other three parties have been allotted shares of favourable legislative districts, setting them up to win at least 15 seats each and form parliamentary groups of their own.

Intra-party statements also bear the language of compromise. To assuage the Europhiles of the Greens and Socialists, LFI has emphasised its firm attachment to the European Union and the eurozone; while the two parties have stressed they’re prepared to sidestep certain EU rules – particularly those that constrain public spending.

Frictions persist. Some of the Socialist old guard is enraged over the deal, including the former president François Hollande, who views it as a betrayal of his legacy. A few Socialists expected to run don’t yet have the formal blessing of the coalition or may not even want it. The Communists have sparred with La France Insoumise over a particular seat in the suburbs of Lyon. Critics have rightfully pointed out the lack of candidates who resemble the working-class and immigrant voters who nearly put Mélenchon in the run-off round last month. Still, the accord looks set to last at least through June’s elections.

While polls show a vast majority of self-identified leftwing voters support the deal, it has also sparked a predictably over-the-top backlash from pundits and political rivals. Le Pen has warned it will lead to prison abolitionist, “pro-burkini” anarchists wreaking havoc in the National Assembly, while Macron and his allies have painted the coalition as an opportunistic cabal led by “far-left” puppet masters who are unfit to govern.

Much of the Nupes programme is, in fact, fairly modest – raising the minimum wage, lowering the retirement age and investing in public services are far from revolutionary proposals – but this is beside the point.

What’s probably more upsetting to the president and his far-right rivals is the political clarification under way. The emergence of a powerful electoral bloc centred on La France Insoumise – a coalition committed to wealth redistribution and the defence of ethnic and religious minorities – would betray a myth that has served both Macron and Le Pen in recent years. It would shatter the illusion that their political camps are the only two options on offer (“progressives” versus “nationalists” or “populists” in Macron-speak; or “patriots” versus “globalists” in the language of the far right).

It has long been apparent that a giant chunk of the French electorate – millions of young people, a swath of the working-class and a sizeable share of middle-class progressives – doesn’t fit within the narrow confines of this debate. Now these voters finally have a coalition that is serious about representing them and taking power.