Barthélemy distanced himself from the PCF in 1930, but eagerly joined French Popular Party (PPF) of Jacques Doriot in 28 June 1936 and quickly rose through the ranks.[2] Disillusioned with Communism, he was attracted to the "revolutionary authenticity" of fascism and National Socialism. In October 1936 he was appointed to the position of the Federal secretary of the party for the Alpes-Maritimes, centered in Nice. He subsequently joined its central committee and its political bureau. In November 1939, he became secretary-general of the party and was installed in Paris.[3] He was a contributor to L'Émancipation nationale ([The National Emancipation]),[4] and Le Cri du peuple.[5]
On 6 August 1944, Barthélemy fled Paris ahead of the Allies' advance. He then took refuge in the Sigmaringen enclave. In November 1944, he was chosen to represent the PPF to the Italian Social Republic. After Doriot's death in February 1945, Barthélemy, Simon Sabiani, and Marcel Marschall formed a triumvirate to head the PPF.
Barthélemy moved to Milan, Italy in April 1945. Arrested on 2 May, he was handed over to French authorities, tried by a military tribunal, and sentenced to several years in prison.
Barthélemy participated in the creation of the National Front of Jean-Marie Le Pen in 1972[7] and was its administrative secretary from 1974 to May 1978.[1]
In 1978, his memoir, Du communisme au fascisme, histoire d'un engagement politique ([From Communism to Fascism: A History of Political Commitment]), was published by Éditions Albin Michel. Philip Rees described it as "perhaps the most interesting and revealing of all the accounts of political conversion from extreme left to extreme right and a vital source book for the history of the PPF."