PPP Chairman Bilawal Bhutto announced his decision after holding consultation with parliamentary leader Hasan Murtaza.
“We have competition with a fascist party, which we cannot support,” said Murtaza.
The bypolls will be held on two empty seats of Senate on November 15. The seats were vacated when PML-N senators Haroon Akhtar and Saadia Abbasi were disqualified due to dual nationality.
The Supreme Court had disqualified the two senators in a hearing of dual nationality case under Chief Justice Saqib Nisar.
The CJP directed the Election Commission of Pakistan to de-notify Saadia Abbasi, the sister of former prime minister Shahid Khaqan Abbasi, and Haroon Akhtar as members of the Parliament in October.