Police confirmed on Tuesday that Hasmin Dra, a former commander of the Moro National Liberation Front (MNLF), had died after an attack last Sunday.
The roadside shooting took place near the southern town of Palimbang on Mindanao island.
Dra was running for a position in the Palimbang town council and was a senior member of the MNLF.
The Front was an Islamist separatist rebel group that has since signed a ceasefire with the government in exchange for the creation of a Muslim autonomous region.
Police suspect the attackers belonged to a local gang called the Abu Sofia group, said military spokesman Lieutenant Colonel Renoir Pascua.
According to a nationwide police tally, at least 71 people have been killed since December in violence related to the 10 May general elections, already surpassing the death toll in the last elections in 1998.