Active in socialist politics, Vaillant was elected to the Chamber of Deputies in 1893, representing the 20th arrondissement. Although he had earlier been a convinced revolutionary, in the Chamber he generally followed a middle ground between the "revolutionaries" represented by Jules Guesde and the "reformists" represented by Jean Jaurès. He was among the founder members of the French Section of the Workers' International (SFIO), a socialist party uniting revolutionary and reformist groups.
