Four other people were killed.
The attack occurred in Bajur, a district in Khyber Pakhtunkhwa province, local police chief Bakhat Bunar said.
He said the former senator, Hidayatullah Khan, had been travelling in a convoy to attend an election rally.
President Asif Ali Zardari and other officials denounced the bombing.
No one claimed responsibility for the attack, and the Pakistani Taliban in a statement said they were not involved.
Bajur until recently was a stronghold of the group, known as Tehrik-e-Taliban Pakistan or TTP.
TTP was emboldened by the Afghan Taliban seizing power in Afghanistan in 2021 as international forces withdrew.
Pakistani authorities often assert that Afghanistan's Taliban rulers are giving shelter to TTP fighters.
The Afghan Taliban government insists it does not allow anyone to use Afghan soil for violence in any country.
Wednesday's attack came about a year after a suicide bomber blew himself up at a political rally of the Jamiat Ulema Islam party in Bajur, killing dozens and wounding 200.
Since then, security forces have conducted operations against insurgents in Bajur and elsewhere.
In the latest operation, security forces killed militant commander Irfan Ullah in Bajur on Wednesday.
In a statement, the military said Ullah was involved in numerous past "terrorist activities" and was behind targeted killings of civilians.
