Often seen as the third-highest leader in the MNR hierarchy (after Paz and Siles), the relatively conservative Guevara clashed repeatedly on ideological grounds with Juan Lechín and others associated with the Left wing of the party.

Fully expecting to be the party's official candidate for president in 1960, he left it abruptly to form his own political organization when Paz Estenssoro decided to return to Bolivia and run for re-election.

In 1964, Guevara supported the military coup d'état that toppled the MNR from power, and once more served as Minister of Foreign Relations, this time to President René Barrientos.

The long years in exile following the establishment of the 1971-78 dictatorship of General Hugo Banzer brought Guevara closer to the main body of the MNR, by now divested of its more left-leaning elements, including Siles and Lechín.

Faced with a mounting economic and fiscal crisis, the new president declared that it might be advisable to extend his mandate by an extra year in order to allow him to confidently take the adequate measures.

Guevara, although bitter by the strange circumstances that surrounded his ousting, resumed his position as president of the Senate on 19 November[1] and continued to support Paz Estenssoro in subsequent elections (1980, 1985).