At 17 years of age Petro became a member of the guerrilla group 19th of April Movement, which later evolved into the M-19 Democratic Alliance, a political party in which he was elected to be a member of the Chamber of Representatives in the 1991 Colombian parliamentary election. He served as a senator as a member of the Alternative Democratic Pole (PDA) party following the 2006 Colombian parliamentary election with the second-largest vote. In 2009, he resigned his position to run in the 2010 Colombian presidential election, finishing fourth in the race.[6]