A leading politician from Pakistan's ruling party was gunned down in a wealthy neighborhood of Islamabad Tuesday by a member of his security detail after speaking out against the country's controversial blasphemy laws, an assassination that highlights the nation's struggle to contain extremism even among those close to the center of power.

Salmaan Taseer, governor of Punjab, Pakistan's most-populous province, and a member of the ruling Pakistan People's Party, died after a member of his security team fired multiple shots into...