The Socialist Party has spent years on the political sidelines since Macron was elected president in 2017, but now holds leverage. Lecornu is meeting with party leaders across the political spectrum to try to get support from a fragmented parliament for his 2026 spending plans, Reuters reported.
Socialist Party leader Olivier Faure said before meeting Lecornu that his party's demands would include "an end to harsh budget cuts, fair contributions from the wealthiest, increased purchasing power".
After the meeting he said Lecornu had not given away anything about what he planned to do.
"We'll see what he says in the coming days," Faure told reporters. "If he's not willing to listen to us, we'll censure him."
The Socialists want Lecornu to clearly commit to adopting key elements of their proposals for the budget, Socialist lawmaker Philippe Brun said. Lecornu's predecessor, Francois Bayrou, was ousted over his own plan for a 44 billion euro budget squeeze.
France is under pressure to lower a budget deficit that stands at nearly double the European Union's 3% ceiling, and a debt pile equivalent to 114% of GDP.
The Socialists' demands include scrapping Macron's unpopular changes to pensions, introducing a new wealth tax and halving the savings proposed by Bayrou for 2026.
An IFOP opinion poll published on Tuesday showed broad public support for the Socialists' proposals, with 86% backing a new wealth tax and 66% supporting a suspension of the pension reform.
But the Socialists face a tough delicate balancing act. If they push too hard, the political crisis could worsen and Macron could be left with no other choice but to call snap parliamentary elections – a scenario that opinion polls suggest would not favor the left.
Brun said the Socialists were open to compromises, since they, like others, don't have a majority in parliament. He added that he reckoned startups like French artificial intelligence Company Mistral AI should be exempted from the proposed wealth tax, to help them grow.
Lecornu has signaled openness to dialogue but faces major constraints, including preserving Macron's legacy and keeping the conservative The Republicains (LR) on board.
Lecornu, a close ally of the president, was picked last week as Macron's fifth prime minister in less than two years after parliament, deeply split between three opposing ideological camps, ousted Bayrou over his budget plans.
Meanwhile, unions and left-wing groups plan nationwide protests on Thursday, demanding action on the budget, but also on wages, pensions, and public services. An Interior Ministry source said that up to 800,000 people were expected to take part in the strikes and protests.