He entered the Cortes in 1854 as a Progressist deputy for Zamora.
After the coup d'Nat of Don Leopold O'Donnell in 1856, Sagasta had to go into exile in France, but promptly returned, to become the manager of the Progressist paper La Iberia, and to sit in the Cortes from 1859 to 1863.
He seconded the Progressist and revolutionary campaign of Prim and the Progressists against the throne of Queen Isabella, conspiring and going into exile with them.
He retired to his house in Logrono, which he left six years later, in 1854, when called upon by the queen to take the lead of the powerful Liberal and Progressist movement which prevailed for two years.
The growing ambition of General O'Donnell constantly clashed with the views of Espartero, until the latter, in sheer disgust, resigned his premiership and left for Logrono, after warning the queen that a conflict was imminent between O'Donnell and the Cortes, backed by the Progressist militia.