The ruling Socialist Party’s first secretary Jean-Christophe Cambadelis urged both politicians on Saturday to take part in what he said would be leftist primaries on January 22 and January 29.
"It is not a leftist primary, it is a Socialist Party primary. I am not a member of the Socialist Party," Melenchon told the France 3 television station.
Melenchon left Socialists eight years ago to found the Left Party together with other dissidents in protest over results of the 2008 party leadership contest.
"Even if this primary did go well, the winner would not succeed," Macron told a French newspaper, Le Journal du Dimanche.
"If Arnaud Montebourg makes it past the primary, you think [Prime Minister] Valls will support him? If Manuel Valls wins, do you think Arnaud Montebourg or Benoit Hamon will rally behind him?" he asked.
The French will elect a successor to unpopular Socialist President Francois Hollande in two rounds of voting in April and May 2017. Center-right The Republicans’ nominee Francois Fillon is expected to face far-right National Front leader Marine Le Pen in the runoff.