Sen. Hillary Clinton, D-N.Y., described Bhutto as a pioneering woman, longtime peer on the world stage and a fellow mother, The Washington Post said.

"I have known Benazir Bhutto for more than 12 years; she's someone whom I was honored to visit as first lady when she was prime minister," Clinton said at an Iowa campaign event, The Post said. "Certainly on a personal level, for those of us who knew her, who were impressed by her commitment, her dedication, her willingness to pick up the mantle of her father, who was also assassinated, it is a terrible, terrible tragedy."

Sen. Barack Obama, D-Ill., began at every campaign stop Thursday with talk of Bhutto.

"She was a respected and resilient advocate for the democratic aspirations of the Pakistani people," Obama said. "We join with them in mourning her loss, and stand with them in their quest for democracy and against the terrorists who threaten the common security of the world."