The 56-year-old LR veteran Ciotti, whose home-base is the right-wing Nice region, says he wants to stop what he calls a "migratory invasion."
"We are together so that France remains France," Ciotti told a rally last month, saying that authority, identity and liberty were the pillars of his policy, with the fight against Islamism a key issue.
LR has lost veteran figures to Macron's camp, including Finance Minister Bruno Le Maire and former prime minister Edouard Philippe.
Nearly three quarters of LR voters consider LR cannot fly solo, an Odoxa poll for LCP showed last month.
The favoured option for LR supporters would be an alliance with Macron's camp, but not far ahead of a deal with the RN.