M’jid El Guerrab, who represents French citizens in Morocco and west Africa, was detained on Friday after Wednesday's violent dispute with Boris Faure, the head of the Socialist Party's federation of French citizens living abroad.
El Guerrab admits having hit Faure twice with a motorcycle helmet.
Faure had to be treated for head injuries in hospital where he remains in intensive care.
El Guerrab has said he was responding to a provocation and an eyewitness told Marianne magazine that Faure called El Guerrab a "dirty Arab", a charge Socialist Party officials and Faure's family dismiss as "laugable".
Resigned from Socialist Party
El Guerrab is a former Socialist, who resigned from the party in order to stand for Macron's party in this year's general election, winning with 60 percent of the vote.
Faure has accused him of being an opportunist who quit the party because he knew he would not be adopted as a candidate.
El Guerrab claims that Faure has been harassing him ever since and has threatened to sue him.
The leader of the Socialist parliamentary group, Olivier Faure (no relation), called on REM to take responsibility for its members and expel El Guerrab.
El Guerrab has taken leave from the REM to allow for the investigation to go ahead unhindered.
Macron's party has welcomed that decision, condemned violence and has summoned El Guerrab to explain his actions at a meeting next week.
Public prosecutors opened an official investigation into assault with a weapon on Saturday afternoon and placed El Guerrab under judicial control.