Ex-French prime minister Manuel Valls has said France's Socialist party "is dead" and he wants to run under the banner of President-elect Emmanuel Macron's political movement in June's parliamentary elections.
Mr Valls, a centre-leaning politician in favour of relaxing labour protections, had already thrown his support behind Mr Macron before the presidential election after losing to Benoit Hamon in the Socialist primary.
Mr Valls told RTL radio that Mr Macron's victory over far-right leader Marine Le Pen in the presidential run-off on Sunday was a blow to populism in Europe, and gave a "terrific" image of France abroad.
Mr Valls said that "I will be a candidate in the presidential majority and I wish to join up to his movement, the Republic on the Move".
However, it is not clear if he will be able to do so.
Mr Valls is planning to run in the Essonne department, his fiefdom south of Paris, but Republic on the Move officials said his nomination will not be automatic.
"Every support to the president is welcome," said Jean-Paul Delevoye, the president of the commission in charge of assessing candidates.
"But support doesn't necessarily translate in nomination. His voice is not insignificant, but his candidacy will be treated as anyone else's."
Mr Hamon, who gained popularity in recent years by leading a group of rebel Socialist lawmakers who opposed Mr Valls, was a distant fifth in the first round of the presidential election after garnering just over 6% of the votes, the Socialist Party's worst result since 1969.
The poor result triggered a fierce debate within the party on the opportunity of sticking to the left platform defended by Mr Hamon, or to switch back to the more centrist views of Mr Valls and his allies.
Guillaume Balas, who co-ordinated Mr Hamon's platform, said Mr Valls "excluded" himself from the party with his allegiance to Mr Macron's movement.
Reflecting on Mr Macron's victory, Mr Valls said: "The old parties are dying, or are already dead.
"I'm not living with regrets. I want Emmanuel Macron, his government and his majority to succeed, for France.
"I will be a candidate in the presidential majority and I wish to join up to his movement, the Republic on the Move."
Mr Macron's 577 candidates in the elections are expected to be announced on Thursday.
He has said he was aiming for an absolute majority in the lower chamber in June's elections. If so, he will be able to choose a prime minister.
If another party wins a majority, Mr Macron could be pressured to choose a prime minister from that party, a situation the French call "cohabitation".
If Mr Macron's party performs poorly, he could also be forced to form a coalition, a common occurrence in many European countries but something very unusual in France.