The Pakistani Taliban claimed responsibility for the attack, a spokesman for the militant group said.
Officials say Ikramullah Gandapur, a local candidate from Imran Khan's party, and his driver were killed by a suicide bomber in the northern city of Dera Ismail Khan, three days before the country's general election.
The nation has been on edge over Wednesday's elections that will decide Pakistan's new prime minister after a string of similar suicide bombings this month against political candidates.
The attack followed a suicide blast in the southwestern province of Balochistan on July 13 which killed 149 people, underscoring continuing security challenges in Pakistan following years of dramatic improvements.
While speaking at the Rawalpindi Bar Association, he accused ISI of pressuring the Chief Justice and other judges to get favourable verdicts in different cases, including Nawaz Sharif's corruption case.
Meanwhile, global community expressed concern as the Pakistan Army is making all efforts to ease cricketer-turned politician and Pakistan Tehreek-e-Insaf (PTI) chief Imran Khan's path to power.
The political involvement of the military and ISI is rarely discussed openly due to fear of repercussions.
The military has warned of threats before the polls, and will put more than 370,000 soldiers on the streets on election day.
Pakistan Muslim League-Nawaz (PML-N) president Shehbaz Sharif has said that people can change his name if he does not take Pakistan ahead of its arch-rival India after coming into power. No one can doubt your Honesty and that's what is require in our country.
Its biggest challenger is the Pakistan Tehreek-e-Insaf, led by former national cricket captain Khan. On the other hand, Pakistan Peoples Party (PPP) will be in action in Sindh and take out rally from Larkana to Shikarpur.