The victim, 45-year-old businessman Jaime Chua, was alone at the campaign headquarters of Mayor Benjamin Lim in Barangay Bonuan Gueset when the gunmen approached him from behind and shot him, according to Superintendent Noel Taliño, city police chief.
Chua died on the spot from four bullet wounds.
Taliño said they have yet to establish whether the killing was politically motivated or personal.
In an interview with dzSD Super Radyo Dagupan, Lim expressed hope that politics was not behind Chuas murder.
"Because if it is, it means there are people who are out to kill people in Dagupan," he said.
"Ag yo ak nataktakot ta abayag ak lan antakot (You cannot frighten me because I have long been afraid)," he said.
He appealed to his political enemies to observe sobriety and let democracy prevail.
Lim said what happened to Chua shocked many people because the businessman was a good person. "I am really sad because this happened to Jaime who was killed like an animal," he said.
The mayor said Chua had intimated that he had received death threats and that he was being followed especially at night.
More than a week ago, three men attempted to torch Lims billboard at the campaign headquarters in Barangay Bonuan Gueset.
Lim is seeking a second term under the banner of Lakas-CMD (Christian-Muslim Democrats). Challenging him are former vice mayor Teodor Manaois of the Koalisyon ng Nagkakaisang Pilipino and Cesar Juinio of the Laban ng Demokratikong Pilipino.
Two other contenders, Tito Perez and Victor Llamas, had withdrawn from the mayoral race and are now running for councilor.