How do you say “landslide” in French? Emmanuel Macron’s new centrist party, La République en Marche – formed just over a year ago – is heading for a true “raz-de-marée” (“tsunami”), the French equivalent to what Theresa May tried, and failed, to secure in the UK general election last week.

And now picture something else. The entire French left is about to disintegrate. It's as if the Labour party disappeared overnight.

With En Marche predicted to win as many as 455 seats out of the 577 composing the Assemblée Nationale, Macron’s victory is complete. A more than comfortable majority in parliament will ensure that his multiple planned reforms pass unchallenged in the house.

Such a parliamentary landslide would be historically unprecedented in French politics. Macron’s party would make up as much as 70 per cent of sitting MPs, according to AFP – the highest ever parliamentary majority for a French president.

After defeating both “traditional” main parties, the Socialists and the Republicans, in the first round of the presidential election, Macron’s landslide victory in parliament may finish them off. The French left, especially, is sinking fast, and if Macron succeeds in bridging “left and right”, it may well be wiped out for a very long time.

In the first round on Sunday night, the Socialist Party came fifth, with the staggering score of 7.44 per cent of the vote – behind En Marche (32 per cent), the Republicans (21.2 per cent), the National Front (13.2 per cent) and Jean-Luc Mélenchon’s hard-left party La France Insoumise (11 per cent).

That no party of the left managed to beat the centre-right or the far right is concerning enough – but in the case of the Socialists, the defeat is crushing.

Just like in the presidential election, where Socialist Benoit Hamon struggled with only 6 per cent of the vote – a sign of the disaster to come – the party’s heavy losses have shown in the first round of the parliamentary vote.

In the North, historically a Socialist territory due to industrial and mining lands, the party has lost all 18 of its MPs in the first round. Twelve of Francois Hollande’s former ministers, including Hamon himself, have been eliminated. Big names such as Cécile Duflot, Aurélie Filippetti, and Emmanuelle Cosse have been voted out – imagine the moment Ed Balls lost his seat but with Yvette Cooper, Andy Burnham and Chuka Umunna losing theirs, too. Le Monde has counted 95 Socialist MPs who have already lost their seats and more could risk theirs in the second round, including Hollande’s former education minister Najat Vallaud-Belkacem.

Macron’s En Marche is wiping out the French left faster than anyone imagined. As for Mélenchon’s Insoumis, they are lagging behind Le Pen’s National Front. They may ultimately win more seats than the far-right in the run-up, but their voice in parliament, and thus of the Socialists, will be a noise in the background.

Both parties refused to make their presidential bid a common cause and opened the way to Macron, who will parade in parliament without a credible opposition. What’s left of them after 19 June will have to unite to survive.