The Socialist Republican Union (USR) was a social democratic political party which was active from 1935 to 1940 during the French Third Republic. It was founded by Marcel Deat as a rally for "socialist republicans", who had been split between the socialist SFIO and liberal Radical-Socialist Party since the 1890s and 1900s. The left-wing republican movement in France failed to form a mass party for years, instead operating independent clubs and small local parties. In 1935, Deat united right-wing splintergroups of the SFIO, left-wing splintergroups of the Radical Party, and the old Republican-Socialist Party form the social democratic "Socialist Republican Union", acting as a midpoint between center-left Radicalism and orthodox Marxist socialism, as well as an electoral vehicle for Deat's right-wing neosocialist movement. In 1936, the USR won 43 seats across France, including three in Paris; however, Deat and many of the party's right-wing members lost their seats, leaving the USR in the hands of its socialist republican majority. It participated in the left-wing Popular Front governments until the outbreak of World War II and the Fall of France, during which Deat and the neosocialists collaborated with Vichy France, while Joseph Paul-Boncour and other USR leaders refused to grant extraordinary powers to Philippe Petain. After the war, the majority of former USR members joined the SFIO, while others joined the parties affiliated with the liberal-progressive Rally of Republican Lefts electoral alliance (i.e. the Radical Party, Independent Radicals, and Democratic and Socialist Union of the Resistance).