The senior vice-president of the party led by Pakistani international cricketer Imran Khan has been shot.
Zahra Shahid Hussain was killed outside her home in the upmarket area of Defence in the southern city of Karachi.
Local reports suggest Hussain, 60, a member of the Pakistan’s Movement for Justice (PTI) party was killed during a robbery, but ex-cricketer Khan has blamed the city’s dominant MQM party.
PTI candidate Dr Arif Alvi was not confident Hussain’s killers will be found.
“I believe the Sindh government should investigate this murder, look for the killers and get them sentenced but unfortunately over the last five years nobody has ever been arrested for the killings to be tried in the court of law,” Alvi said.
Hussain’s death comes on the eve of a partial rerun in Karachi of last weekend’s general election after her PTI party accused MQM of electoral fraud.