Leighton was one of the founding members of the Chilean Christian Democrat Party. In 1966, he entered the government of Eduardo Frei Montalva, becoming a vice-president. Leighton left Chile in December 1973, after the coup d'état. In exile, he opposed the new regime of Pinochet, he had already been nominally open to Salvador Allende. In 1974, a year before the Leighton assassination attempt, General Prats, another Chilean exile, was murdered in the capital of Argentina, Buenos Aires.