Militants targeted a gathering of Balochistan Awami Party (BAP) leader Siraj Raisani in Mastung area in the troubled Balochistan province, PTI reported.
"Mir Siraj Raisani succumbed to wounds while he was being shifted to Quetta", he added.
Officials earlier said at least 40 people had been injured.
Islamic State claimed responsibility for the attack, the group's AMAQ news agency said.
Earlier, Agha Umar Bungalzai, provincial home minister, told AFP that the explosion in Mastung town, near the provincial capital of Quetta, killed Siraj Raisani, who was running for a provincial seat with Balochistan Awami Party (BAP). "We have more than 200 people injured and under treatment at different hospitals in Quetta and condition of some of the wounded is critical", he said.
The officials have said the death toll has climbed at least four people while at least 32 others have sustained injuries and are undergoing treatment in the District Headquarter Hospital in Bannu.
The death toll from a suicide bombing at an election rally in Pakistan's southwest has risen to 128, a government official has confirmed. They said that around 16-20 kilogram of explosives were used in the attack. Five persons were killed and over 37 other injured, police said.
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Durrani came under attack in 2015 as well.
Pakistan's caretaker government launched a crackdown on political gatherings on Friday as Sharif, who was ousted by the Supreme Court past year and convicted in absentia of corruption a week ago, flew back to rally his party ahead of the general elections.
Mr Durrani, a leader of the Jamiat-Ulema-i-Islam-Fazl (JUI-F) - who had served as a federal minister in the Pakistan Muslim League-Nawaz-led government and as chief minister of KP, then North Western Frontier Province, in 2002-07 - was in Hoveda, some 16km south of Bannu city, in connection with his election campaign.
President Mamnoon Hussain and caretaker Prime Minister retired Justice Nasirul Mulk have strongly condemned blast in Mastung, Balochistan.
On Tuesday, a bomb claimed by the Taliban targeted a rally by the Awami National Party (ANP) in the city of Peshawar.
Pakistani Taliban have been targeting the members and the leaders of Durrani's party in the past for the religious group's support and participation in the democratic process, which the militants say is a system brought in by Western infidels.
The Islamic State group has a muted presence in Pakistan but has carried out brutal attacks there in the past, including the blast at a Sufi shrine in February previous year which killed almost 90 people.