The PSD's representation in the Brazilian Chamber of Deputies steadily shrunk from 52.8% of the seats in 1945 to 28.9% in 1962. By 1963, the Labour Party, PSD's minor opposition party, had more seats than the PSD.
A considerable faction within the PSD turned against President João Goulart, a PTB member who had been Vice President until he stood in for retired President Jânio Quadros in 1961, whom they deemed to be too leftist. They therefore supported the military coup d'état on 1 April 1964, making an important contribution to the success of the overthrow.