He also led the founding congress of the Central Obrera Boliviana (COB), an umbrella federation of labor unions, and was elected its Executive Secretary.

Since he had played a vital role in the Revolution, and had advocated the permanent extension of weapons to the workers' militias to guarantee regime stability against the possibility of an oligarchic/military backlash, he became extremely popular with the poorer sectors of society.

In frank disagreement with what he saw as the increasingly conservative policies of president Hernán Siles Zuazo, by the late 1950s Lechín had begun to form a left-wing opposition within the ruling party.

Once more Lechín's role was polarizing, as he attempted to create a parallel (union and Assembly-based, quite reminiscent of soviets) alternative to the established order.

In his restored position as top labor leader in the country, he strongly criticized the economic policies of president Hernán Siles Zuazo (1982–85), coming close to toppling his beleaguered regime with crippling strikes and other non-cooperation measures.

The closure of most of the country's tin mines by Paz Estenssoro (due to declining production and the collapse of world prices) led to considerable in-fighting in the unions.

A major criticism used against him is that he tended to de-stabilize precisely those regimes that were friendliest to the interests of the working class (Paz Estenssoro, Torres, Siles Zuazo between 1982 and 1985), while being quite deferential to the most hardline right-wing governments (Barrientos, Banzer, Garcia Mesa), at least until he could get safely out of the country.