In September 1792, Couthon was elected to the National Convention. During a visit to Flanders, where he sought treatment for his health, he met and befriended Charles François Dumouriez and later writing praises of him to the Assembly and referred to him as "a man essential to us." His relationship with Dumouriez briefly caused Couthon to consider joining the Girondist faction of the Assembly, but after the Girondist electors of the Committee of the Constitution refused Couthon a seat on the Committee in October 1792, he ultimately committed himself to the Montagnards and the inner group formed around Maximilien Robespierre; both shared many opinions. Couthon became an enthusiastic supporter of the Montagnards and often echoed their opinions. At the Trial of Louis XVI in December 1792, he argued loudly against the Girondist request for a referendum and went on to vote for the death sentence without appeal. On 30 May 1793, Couthon was appointed as secretary, on 10 July as member to the Committee of Public Safety, where he would work closely with Robespierre and Saint-Just in the planning of policy strategy and policing personnel. Three days after rising to that position, Couthon was the first to demand the arrest of proscribed Girondists.