ISLAMABAD, Pakistan — The assassination of an outspoken secular politician by one of his elite police guards on Tuesday plunged the government deeper into political crisis and highlighted the threat of militant infiltration even within the nation’s security forces.

The killing of Salman Taseer, the prominent governor of Punjab Province, was another grim reminder of the risks that Pakistani leaders take to oppose religious extremists, at a time when the United States is pushing Pakistan for greater cooperation in the war in Afghanistan by cracking down on militant groups like the Taliban.

Mr. Taseer, 65, a successful businessman and publisher of a liberal English-language daily newspaper, was exceptional, even within the secular-minded Pakistan Peoples Party, for his vocal opposition to the religious parties and the extremism they spread. He was imprisoned in the 1980s under the military dictator Gen. Mohammad Zia ul-Haq for it and was still opposing the religious parties 30 years later.

He recently took up a campaign to repeal Pakistan’s contentious blasphemy laws, which were passed under General Zia as a way to promote Islam and unite the country. The laws have been misused to convict minority Pakistanis as the Islamic forces unleashed by the general have gathered strength. The laws prescribe a mandatory death sentence for anyone convicted of insulting Islam.