In 1940 Siles was elected in the Chamber of Deputies. Gravitating toward the reformist side of the political spectrum (even though his father had been one of the pillars of the Old Regime), in 1941 he founded along with
Víctor Paz Estenssoro and others, the influential
Revolutionary Nationalist Movement (Movimiento Nacionalista Revolucionario, or MNR). The MNR was behind the coup that installed the progressive military administration of
Gualberto Villarroel (1943–1946), but was forced from power due to U.S. pressure and also by Villarroel's overthrow in 1946 after which Siles was exiled to Argentina. There he worked as a correspondent for
Associated Press from November 1947 until September 1948. In 1949, Siles returned to Bolivia to stand for election to the Bolivian legislature, which he won. However, he was soon arrested for continued agitation against the government in power and was sent to the island prison of
Isla de la Luna (Coati) on lake
Titicaca. In September 1949, he escaped from the island prison together with a number of fellow political prisoners and, with Bolivian police in hot pursuit, was able to reach Peru, where he was granted political asylum. On 9 October 1949, he returned clandestinely to Bolivia and appeared in the Bolivian congress at his assigned bench, where he demanded his back pay—this public relations coup cemented his image as a leader of the resistance against a tyrannical government. In the
1951 elections Paz Estenssoro ran for presidency with Siles as his vice-presidential running mate, and won the contest with 42.9% of the vote. However, the ultra-conservative government of
Mamerto Urriolagoitía refused to recognize the results and instead turned over the presidency to the commander of the Bolivian army, general
Hugo Ballivián. At this point, the MNR party went underground and from 9–11 April 1952 led the historic
Bolivian National Revolution, aided by defections from the armed forces to the rebel cause (key among which was general Antonio Seleme). Siles played a major role in the revolutionary uprising, along with
Juan Lechín, since the MNR leader Paz Estenssoro was at the time in exile in Argentina. == Vice President (1952–1956) ==