Complex sentence: Alfonsín was elected to the Chamber of Deputies in 1963, becoming one of President Arturo Illia's most steadfast supporters in Congress; he lost his seat when a military coup removed Illia from office in 1966.
Split into simpler sentences: Alfonsín was elected to the Chamber of Deputies in 1963, becoming one of President Arturo Illia's most steadfast supporters in Congress. He lost his seat when a military coup removed Illia from office in 1966.