The Socialist Party of France and Jean-Luc Mélenchon's hard-left La France Insoumise (LFI) party have agreed in principle to ally for the June legislative elections.

The coalition agreement reached earlier this week by the Greens and Communists is an attempt to deny Emmanuel Macron a parliamentary majority and stymie his pro-business agenda after he was re-elected president in April.

“We can and will beat Emmanuel Macron and we can do it with a majority to govern for a radical program,” the LFI lawmaker Adrien Quatennens told Franceinfo radio.

If the LFI-Socialist accord is approved, the French left will be united for the first time in 20 years.

The agreement was crafted under Mélenchon's leadership, who split from the Socialist party in 2008 due to its pro-European Union stance, wanting to "disobey" the bloc's laws on budget and competition issues, as well as question its free-market ideals.

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According to a source within the Socialist Party (PS), there was agreement on who would run in which constituency and on overall strategy, but negotiators still needed to clarify elements of the joint program itself.

Social and Ecological People's Union

Reports stated that the wording of the platform for the new alliance, which will run under the banner of the "Social and Ecological People's Union," in Europe is still being contested.

The deal would then need to be approved by the PS’s national committee.

The new alliance's policies include promises to cut the retirement age to 60, raise the minimum wage, and restrict pricing on vital goods.

If confirmed, Mélenchon's accomplishment in reaching an agreement with the Socialists would signal a watershed moment for a party that has given the country two presidents since World War II and has been a driving force for European integration.

A source close to the talks said the agreement – in which only one candidate from any party that joins the alliance runs in any of France's 577 constituencies - foresees the PS having only 70 candidates in mainland France, and potentially a few more in overseas territories.

According to a recent Harris Interactive poll, the left is unified and an alliance between Macron's party and the conservatives is neck and neck, with each receiving 33% of the parliamentary vote. However, forecasts show that given France's two-round election system, Macron might still win a majority of seats.