By Sudip Kar-Gupta and Elizabeth Pineau

PARIS (Reuters) -France’s Socialist Party sealed a deal on Friday to join the French left’s first broad coalition pact in 20 years, in a bid to deprive newly re-elected President Emmanuel Macron of a majority in June’s parliamentary elections.

The pact, which the Greens and Communists already approved earlier this week, sees the eurosceptic, hard-left La France Insoumise (LFI) in the driving seat for the first time, in a deal that is shaking up French politics.

“With the New Popular Union, we can win the legislative elections and change people’s lives!” LFI EU lawmaker Manuel Bompard, referring to the name of the new leftwing alliance.

Macron won a second presidential mandate last month, but he will need a majority in parliament if he is to push through policies including raising the retirement age to 65 from 62 currently, opposed by both the left and far right.

Pollsters’ initial projections show Macron’s party – which was renamed Renaissance on Thursday – on track to win the June 12 and 19 ballot. But the surveys were carried out before the left-wing alliance was struck, and before individual candidates were chosen by each camp for France’s 577 constituencies.

The leftwing deal has taken shape under the leadership of LFI’s firebrand chief Jean-Luc Melenchon, who narrowly missed out on making it to the final round of the French presidential election run-off vote last month.

Melenchon came third with around 22% of votes in the first round of the election, just behind far-right candidate Marine Le Pen. The Socialists’ presidential candidate, Anne Hidalgo, only claimed around 1.7% of votes.

The Socialist Party, long the dominant force on the left and a driver of European integration, is now reduced to a subordinate role, triggering much disquiet within the party. Some veterans have quit.

With Macron’s newly renamed Renaissance party also in the process of fielding candidates for the legislature, Stanislas Guerini, a senior party member, urged Socialists disappointed by their party’s deal with the hard left to join them.

“When I see the Socialist Party … abandoning its (pro-European) beliefs for a few constituencies, I tell social democrats: join us!”, he said on RTL radio.

In recent French legislative ballots, the president’s party has always won a majority in parliament.

Should the outcome be different this time, Macron would have little choice but to name a prime minister from another party, ushering in what has traditionally been a tense period of “cohabitation” during which presidential powers are curbed.

(Reporting by Camille Raynaud, Sudip Kar-Gupta, ELizabeth Pineau, Ingrid Melander; Editing by Kenneth Maxwell and Toby Chopra)