The MSR supported the return of Pierre Laval to the Vichy government of Petain, after he was removed from government in December 1940. They collaborated with the Rassemblement National Populaire (RNP), which was founded in January 1941 and of which the MSR became a faction.
A split in the RNP came after the Eastern Front opened up in July 1941 and the LVF was formed. Another frontman in the RNP was Marcel Déat, who had the confidence of Laval. When he found out Deloncle was plotting against him, he had him and his faction removed from the RNP. Deloncle also took many member of the RNP's paramilitary wing with him.
In October 1941, Deloncle plotted against seven Parisian synagogues with the help of a local SS officer, Hans Sommer, who provided the explosives for the attack.
Further splits in the MSR happened over the next year, as Deloncle became more occupied with the LVF. The other factions at that time were based around Jean Filliol, a former Cagoulard, and the revolutionaries around Georges Soulès and André Mahé. A coup against the Deloncle faction was completed on May 14, 1942, which left Deloncle without a political future. He was killed two years later in a shootout with the Gestapo, who suspected him of having obtained ties to the Allies.
Filiol saw himself plotting against Laval, whose government interned him in October of 1942. The remaining faction of the MSR around Soulès moved into an anti-German position, but would have completely disappeared at the end of the war.