A fervent Roman Catholic, he devoted himself to advocating a patriarchal type of Christian Socialism. His was the most prominent and eloquent member of the Cercles Catholiques d'Ouvriers, and his attacks on Third French Republic's social policy ultimately provoked a prohibition from the Minister of War. He thereupon resigned his commission (November 1875), and in the following February stood as Royalist and Catholic candidate for Pontivy.
The influence of the Church was exerted to secure his election, and, during the proceedings, he was awarded the Order of Saint Gregory the Great by Pope Pius IX. He won the next elections for the same area, but the result was declared invalid. De Mun was re-elected, however, in the following August, and for many years was the most conspicuous leader of the anti-Republican party.
But as a faithful Catholic he obeyed the modernising encyclical of 1892, Rerum Novarum, and declared his readiness to rally to a Republican government, provided that it respected religion. In the following January he received from Leo XIII a letter commending his action, and encouraging him in his social reforms.
He was defeated at the general election of that year, but in 1894 was elected in Finistère (Morlaix). In 1897 he succeeded Jules Simon as a member of the French Academy, owing to the quality and eloquence of his speeches, which, with a few pamphlets, form the bulk of his published work. In Ma vocation sociale (1908) he wrote an explanation and justification of his career.