The Radical Party was the dominant political force of the French Third Republic from about 1900 onward, but declined continually during the postwar period - ending up in the state we see it today, divided and incapable of surviving without the support of the PS and UMP.
Your challenge is to create a scenario, with a POD no earlier than the establishment of the Fourth Republic in October 1946, and which does not involve the premature death of De Gaulle (feel free to get rid of him in another way), where the Radical Party is one of two major political parties in modern France. Note that it may be either the main center-right party or the main center-left party; I can think of a plausible scenario for each outcome.