He began his political career as an activist of the Institutional Revolutionary Party (IPR), which ruled Mexico for over 70 years (1929-2000). He was also the director of the Indigenous People’s Institute of Tabasco. Most probably it was at this time that he became interested in the fate of the poor.
In 1989 he joined the Party of the Democratic Revolution, which was formed after the split of IPR. Five years later he was appointed governor of Tabasco State. In 2000, he was elected mayor of Mexico City. Despite his radical views, he did not do anything disturbing during his term of office: built roads, introduced a small universal pension for the elderly without social security, prepared a support program for the disabled and provided free school supplies for children in the public schools.
In 2006 López Obrador lost the presidential election to Felipe Calderón by 240,000 votes. He said that the elections had been rigged by the government and proclaimed himself a legitimate president. His supporters for six weeks blocked Paseo de la Reforma, the main street in the capital, by organising protests and marches. During this time, López Obrador discouraged a large number of businesspeople.