Alfonsín was elected deputy for the legislature of the Buenos Aires province in 1958, on the UCRP ticket, and was reelected in 1962. He moved to La Plata, capital of the province, during his tenure. President Frondizi was ousted by a military coup on 29 March 1962, which also closed the provincial legislature. Alfonsín returned to Chascomús. The UCRP prevailed over the UCRI the following year, leading to the presidency of Arturo Umberto Illia. Alfonsín was elected a national deputy, and then vice president of the UCRP bloc in the congress. In 1963 he was appointed president of the party committee for the province of Buenos Aires.