Mr Khan's party appears to have surged in popularity in recent months, leaving the election as a two horse race with the Pakistan Muslim League (PML-N) party of jailed former prime Minister Nawaz Sharif.
In another political attack Sunday in the northwestern town of Bannu, unknown assailants fired at a vehicle carrying Akram Khan Durrani, leader of the Jamiat-Ulema-Islam-Fazl party that supports the Afghan Taliban. "It is a big conspiracy against the state of Pakistan", he added.
"Ikramullah Gandapur has been killed and we are conducting a post-mortem", said police official Zahoor Afridi.
On the other hand, the election for PK-99 has been postponed, the Election Commission announced shortly after Gandapur was killed with his driver in a bomb blast near their vehicle in Dera Ismail Khan Sunday.
The Pakistani Taliban claimed responsibility for the attack, a spokesman for the militant group said.
In 2013, Mr Gandapur's brother, Israrullah, who was then the provincial law minister, was killed in a suicide attack on his home.
However, security agencies point finger at Pakistani Taliban who have always been targeting the security agencies and civilians through suicide bombings. Abbasi was sentenced in a rare late-night session Saturday.More news: Breaking down the Spurs-Raptors blockbuster trade
The six-year-old case was related to the supply of ephedrine to a drug smuggler. Imran Khan has emerged as a candidate with right blend of fundamentalist politics and military compliance.
Siddiqui had also claimed that the ISI had approached him with an offer to make him the chief justice of the Islamabad High Court.
Following Sunday's attacks, Pakistan's election oversight body postponed the balloting for the provincial assembly seat that Gandapur was contesting.
"You have got what it takes Skipper Imran Khan, but it will take everything you have.No one can doubt your Honesty and that's what is required in our country.An honest LEADER". His brother Shahbaz Sharif is leading the charge of the party in his absence.
Why India should care Ties with India are probably one of the more important issues plaguing Pakistan ahead of the polls, and is the most complex part of each of the two countries' foreign policy.
Bilawal Bhutto Zardari of Pakistan People's Party (PPP) is another player in the election.