The Attorney General has initiated an investigation and photos of the vehicle used in the attack are in circulation.
Police say the politician was driving with his wife and children down a highway when they were intercepted and forced to pull over by a white car. The male drivers shot Marquez and disappeared.
Paramedics arriving at the scene tried to revive him to no avail. His family was shaken, but unharmed.
The death comes just five days after the politician launched his campaign for mayor of San Juan Colotepec under the banner of Mexico's National Regeneration Movement (Morena) party, the party of current President Andres Manuel Lopez Obrador. This killing brings the number of members from the leftist party in Oaxaca to three since January, following Cutberto Porcayo of San Jose del Progreso and Alejandro Aparicio of Tlaxiaco.
Since July 2018, at least 8 leaders from the Morena party have been killed following the last elections. A party leader, Yeidckol Polevnsky indicated in January that 37 others from her party have been killed in the last five years.
The attorney general has initiated an investigation and photos of the vehicle are in circulation.
Mexico's political sector often falls victim to deadly violence. The consulting firm Etellekt, which specializes in risk, political violence, and public policy reported 175 politicians murdered between September 2017 and August 2018 in Mexico. Of the total 850 attacks registered, 81 percent were against opposition party members.
Etellekt registered a record number of attacks during the past elections, with 48 candidates and precandidates murdered.
The states with the highest rates of poverty had also the highest violence levels.