Center (COB) leader, Juan Lechín. Paz Estenssoro's temporary retirement and polarization of the MNR (1956–1960) Paz was not allowed to run for another consecutive term, and Hernán Siles was elected, serving as President from 1956 until 1960. During the Hernán Siles administration, the MNR began to polarize and fragment, with a conservative wing led by Wálter Guevara and an increasingly assertive left-leaning faction commanded by the charismatic COB leader Lechín. To prevent the fracturing of his party, Paz returned from London (where he had been serving as Bolivian ambassador) and ran for re-election in 1960, winning with an ample majority of the votes. His choice as vice-presidential running mate was the increasingly hard-to-manage Juan Lechín, an action that prompted the defection from the MNR of Wálter Guevara, who felt he had been stepped over. Second and third Paz Estenssoro governments, 1960–1964 The second Paz Estenssoro administration was plagued by violence, dissent, and continued hemorraghing of the original leadership. Of great importance during this period was the thorny issue of disarming the miners and workers' militias who had combatted in the 1952 Revolution and who had been for the most allowed to keep their weapons since. They had served as a useful counterbalance to the possibility of a conservative or military reassertion against the Revolution, but by 1960 were serving the interests of the party's radical left vice-president Lechín. Of Marxist political persuasion, the latter opposed the disarming of the militias and the reconstitution of the traditional military, urged the passing of more far-reaching reforms. Paz Estenssoro disagreed and, continuing the policies started by Siles, increasingly leaned on the "new" armed forces for support. This produced the inevitable final rift, and Lechín was expelled from the party prior to the 1964 elections. Increasingly unable to control events, and considering himself the only man who could keep the MNR coalition together, in 1964 Paz decided to amend the constitution to allow himself to run for re-election. Traditionally, attempts such as these (known as "prorroguismo") have been strongly condemned by the Bolivian political elites, many of whose members may have been waiting for their turn to occupy the presidential palace for years. This was no exception, and Paz's move ended up being his undoing. To symbolize Paz's steady rightward drift, he chose the charismatic commander of the Bolivian Air Force, General René Barrientos, as his running mate. To be fair, Paz's increased reliance