ISLAMABAD, Pakistan — A prominent secular political party on Wednesday accused Pakistan’s powerful security establishment of assassinating one of its former lawmakers a day earlier.

Syed Ali Raza Abidi, 46, was killed in a drive-by shooting on Tuesday evening outside his home in Karachi, a volatile city that is Pakistan’s commercial and economic hub. The party he once represented, Muttahida Qaumi Movement, called it a “coldblooded assassination” that was “part of a crackdown on the party by Pakistan’s military establishment.”

The Muttahida Qaumi Movement, or M.Q.M., controlled the politics of Karachi for decades, but has come under increasing pressure in recent years after a crackdown by security forces. The party’s founding leader, Altaf Hussain, has lived in self-imposed exile in London for the past two decades.

Critics said M.Q.M. maintained iron-fisted control over the city through fear and intimidation with a network of armed enforcers. The crackdown by security forces dismantled much of the party’s militant networks. The M.Q.M. fragmented into several factions, with each group vying for political legitimacy and control over Karachi.