Two senior members of the Awami National party were among those named.
Abbas, a former provincial government minister, was accused of involvement in the killing of a member of the rival family and two other people in an election clash in 1997 but he was never convicted.
"Abbas and his nephew were riding a motorcycle and as they reached a deserted place the assailants, who were probably chasing them in a car, opened fire and killed them both," Faraz Khan, a police officer said on Monday.
Regional rivalries in Pakistan often affect politics and elections. A general election is due late this year or early next.