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.
"Ikramullah Gandapur has been killed and we are conducting a post-mortem", said police official Zahoor Afridi.
Pakistan's top court on Sunday said it had begun reviewing statements made by a judge alleging the country's spy agencies were influencing judicial proceedings, as the powerful military called for an investigation. The Pakistan Taliban claimed responsibility for the attack. Along with his driver who was also killed, three other people, including two policemen, were wounded.
Violence in Pakistan has dropped significantly since the country's deadliest-ever militant attack, an assault on a school in the northwestern city of Peshawar in 2014 that killed more than 150 people, a lot of them children.
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After his younger brother's death, Gandapur was elected as a member of the assembly of Khyber Pakhtunkhwa province and appointed provincial minister for agriculture.
Its biggest challenger is the Pakistan Tehreek-e-Insaf, led by former national cricket captain Khan.
The convoy of another election candidate, Akram Durrani, also came under fire in the northwestern town of Bannu, though no one was hurt.
Ikramullah Gandapur, a member of Mr Khan's Pakistan Tehreek-e-Insaf party, was contesting polls for the Khyber-Pakhtunkhwa provincial assembly when he was killed by a auto bomb.
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Earlier, a suicide blast at an election rally on July 10, in Peshawar, Khyber Pakhtunkhwa's capital, had killed 21 people including election candidate.
The leader of a conservative political party alligned to former Pakistan prime minister Nawaz Sharif has pledged to take Pakistan ahead of its arch rival India, saying he would emulate the leadership of Dr Mahathir Mohamad.
Voting for those two seats would take place at a later date.
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