[2] Martínez consequently founded and led the Republican Union and participated in the Spanish Popular Front,[3] being elected to government in 1936.

He led the integration of the Republican Union into the Popular Front, being elected the speaker of the Cortes (Spanish Parliament).

[4] In February 1939, he rejected to replace Manuel Azaña as president of the Republic.

[5] Following the resignation of Santiago Casares Quiroga two days after the outbreak of the civil war, he was appointed prime minister on 19 July 1936.

As part of his intention to avert war, his cabinet ignored the left wing of the Popular Front, but he would last just a few hours, and he resigned later the same morning, after an unsuccessful appeal to Nationalist General Emilio Mola to avoid war, and was succeeded by José Giral.