In 1942, Laurendeau entered into politics in opposition to conscription, as a member of the "Ligue pour la défense du Canada". His primary reason for doing so was that Prime Minister Mackenzie King had promised conscription would not become national policy, only to put a plebiscite to Canadians to determine whether or not he might revoke his promise and retain their favour. Laurendeau later took part in the founding of the centre-left party "Bloc populaire Canadien", and soon became its provincial leader while Maxime Raymond was its federal leader. Laurendeau was a Member of the Legislative Assembly of Quebec (MNA) from 1944 to 1948 in Montréal-Laurier electoral district.