Phang Nga police went to the scene after receiving an emergency call and found a Chevrolet pick-up truck with a dead body inside.
The dead man was identified as Sophon Kaosoo, 50, a Laem Kaen Pattana Party candidate in the local council elections, due to take place on March 2.
Officers found a bullet casing at the scene which they identified as being 12mm in calibre.
Witnesses said that Mr Sophon was riding in the passenger seat of the truck, which had been mounted with campaign posters and a sound system in the bed. Driving was 40-year-old Adisorn Raksajan.
They were overtaken by a car with no number plates that had been following behind. As the car overtook a gunman opened fire from it, putting at least nine bullets through the windscreen and hitting both men.
Mr Sophon died immediately. Mr Adisorn was rushed towards the nearest hospital but died on the way.
Police believe that the assassination was most likely politically motivated and are trying to identify and locate the two people in the assassin’s car.
Laem Kaen is just to the south of Khao Lak, close to Tab Lamu Royal Thai Navy base.