"This type of loss of life points out again the need for our nation and other civilized nations of the West and Muslim world to come together to support moderate Islamic leaders and moderate Islamic people to help them in their effort to reject the violence and the extreme," Romney told reporters after his first campaign event at Norton's Classic Cafe, in Nashua, N.H. "The world is very much at risk by virtue of these radical, violent extremists and we must come together, in great haste and great earnestness, to help overcome the threat of the spread of radical, violent jihad."

On Wednesday night, Romney had criticized Pakistan President Pervez Musharraf for imposing martial law. Romney was asked if he had more sympathy for such action given the attack on Bhutto.

"I believe it was a mistake. I believe as well that martial law was principally imposed by him to protect himself from political challenge, a challenge from the Supreme Court and others, and believe that it was not a productive course for his nation," Romney said.

Obama said he was shocked and saddened by Bhutto's death.

"She was a respected and resilient advocate for the democratic aspirations of the Pakistani people. 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," he said.

Republican candidate Mike Huckabee said the assassination was "devastating news for the people of Pakistan, and my prayers go out to them."

"The terrible violence surrounding Pakistan's upcoming election stands in stark contrast to the peaceful transition of power that we embrace in our country through our Constitution," Huckabee said in a statement.

Democratic New Mexico Gov. Bill Richardson, a former U.S. ambassador to the United Nations, called on President Bush to force Musharraf to step down. Until then, Richardson said the U.S. must suspend military aid to the Pakistani government.

"A leader has died, but democracy must live. The United States government cannot stand by and allow Pakistan's return to democracy to be derailed or delayed by violence," Richardson said.

The Bush administration has pushed hard for peaceful elections in Pakistan, a nuclear-armed nation led by Musharraf, a U.S. ally in the anti-terror war.

Bhutto was killed in an attack on an election rally in Rawalpindi. Many others were killed in a blast that took place as Bhutto left the rally where she had addressed thousands of supporters in her campaign for Jan. 8 parliamentary elections.

Bhutto served twice as Pakistan's prime minister between 1988 and 1996. She had returned to Pakistan from an eight-year exile Oct. 18. Her homecoming parade in Karachi was also targeted by a suicide attacker, and more than 140 people were killed. On that occasion she narrowly escaped injury.