Investigation showed that the victim, Junie Ablites, 35, was driving a single motorcycle when a white Toyota pick-up blocked his path.
Ablites was said to be on his way to a meeting of supporters of Lakas-CMD mayoral bet Doandre Ladaga when he was attacked.
One of two men alighted from the pick-up and aimed a gun at Ablites. The latter jumped out of his motorcycle and ran, but the man chased and shot him in the upper left shoulder.
The gunman hastily left, while Ablites was immediately rushed to the Loreto District Hospital and later transferred to the Caraga Regional Hospital in Surigao City, a one-hour boat ride from Dinagat Island.
Police later identified the Toyota pick-up with license plate GED 551 as allegedly owned by mayoralty candidate James Gupana.
Chief Superintendent Antonio Dator, Caraga police director, has ordered the deployment of an augmentation force in Loreto following the ambush.