After talks dragged through the night past a Tuesday deadline, the Socialist Party (PS) fell in line alongside the Greens and the Communist Party (PCF) behind the hard-left France Unbowed movement (LFI), who emerged as the dominant force on the left in April’s presidential election.

“We want to elect MPs in a majority of constituencies to stop Emmanuel Macron from pursuing his unjust and brutal policies and beat the far-right,” the PS and LFI said in a joint statement.

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The alliance must still be approved by the Socialists’ National Council on Thursday, with people close to the party leadership warning against viewing the vote as a foregone conclusion.

The negotiation was “historic”, LFI European MP Manon Aubry told Europe 1 radio, offering each party autonomy within “a common policy framework”.

PS heavyweights like François Hollande, in power just five years ago before the Socialists’ precipitous fall from grace, have made clear their opposition to the deal.

He has warned the left-wing tie-up could amount to the “disappearance” of the Socialists.

But Aubry said Wednesday that it was exactly their “important steps concerning Hollande’s legacy” that had allowed the Socialists to align with the rest of the left.

Behind the euphoria at overcoming the traditionally fragmented French left’s differences, the junior partners are eyeing how constituencies will be parcelled out between the parties, with each aiming to run on the united ticket in a maximum of “winnable” seats.

The final deal looks set to distribute around 100 constituencies to the Greens, 70 for the Socialist Party and 50 for the Communist Party.

With some Socialists refusing to stand down, former minister under Hollande Stephane Le Foll said he was ready to lead them in a separate campaign.

At least one Communist candidate, in a suburb of eastern city Lyon, has also said she will not make way for an alliance-backed replacement.