It was founded to support François Mitterrand's candidature at the 1965 presidential election and to counterbalance the Communist preponderance over the French left.

The Socialist Gaston Defferre proposed the creation of a "Great Federation" gathering the center-left and the center-right parties in order to resist to the Gaullist domination over the country and to the leading role of the French Communist Party (PCF) over the opposition.

François Mitterrand then proposed to be candidate with a strategy of union between the left-wing forces, including the PCF.

Nevertheless, in order to talk on an equal footing with the Communists, he advocated the constitution of a federation of the left-wing non-Communist forces.

Finally, de Gaulle dissolved the National Assembly and his followers won the June 1968 legislative election.