Braulio Zaragoza, head of the National Action Party in Guerrero state, was shot three times in the back by the three assassins at the restaurant of the cliffside El Mirador hotel, Guerrero state prosecutors said.
The national leader of the National Action Party (PAN), Gustavo Madero, condemned the killing and demanded a thorough investigation.
Local politicians often have been targeted by threats and violence Mexico launched a military offensive against drug cartels under former president Felipe Calderon, a PAN member, in 2006.
More than 80,000 people have been killed in drug-related violence in the past eight years.