The assassination of former Pakistani Prime Minister Benazir Bhutto forced presidential candidates to shift focus from the domestic front toward foreign policy Thursday, as the attack dashed any hopes on the campaign trail for a scripted finish in the days leading to the Jan. 3 Iowa caucuses.

The tragedy posed both an opportunity and a challenge for candidates to demonstrate to Iowans how they would handle an overseas crisis.

The Democratic candidates called for Pakistan to move firmly toward democracy Thursday despite the unrest. Former North Carolina Sen. John Edwards, whose populist campaign has been largely focused on domestic issues, even spoke with Pakistani President Pervez Musharraf over the phone Thursday afternoon.

"I urged him to continue the democratization process because of how important it is to the Pakistani people and how important it is to his country," Edwards told supporters in Decorah, Iowa. "I also urged him to allow international investigators into Pakistan ... I believe this is the time for America to be a strong and calming influence in a difficult and unstable environment."

Meanwhile, Republican candidates used the attack to bolster their argument that terrorism is pervasive and global and must be fought aggressively.

"This points out again the extraordinary reality of global, violent, radical jihadism," former Massachusetts Gov. Mitt Romney said at a press conference in New Hampshire. "There's no question that the violence that we see throughout the world is violence that is not limited to ... Iraq and Afghanistan, but is more global in nature."

"The world is very much at risk by virtue of these radical violent extremists, and we must come together in an effort in great haste and with great earnestness to help overcome the threat of the spread of radical, violent jihad," he said. Romney on Wednesday night had criticized Musharraf for imposing martial law in the country.

Rudy Giuliani, whose performance as New York City's mayor during the Sept. 11, 2001, attacks has shaped his image on the campaign trail, was the first to issue a statement Thursday. He folded the tragedy into the anti-terror message that he already had revived in a new ad campaign where he invokes the 2001 attacks.

At a campaign stop in Florida, Giuliani called for the the United States to "redouble our efforts in that area of the world, in Pakistan, Afghanistan, and make sure the successes that they brought about -- our military brought about -- in 2001, 2002, become permanent, and there isn't a slip back into terrorist control into that region."

He told FOX News that the attack "underscores for us why we have to remain on offense with Islamic terrorism."

Arizona Sen. John McCain echoed the sentiments of his rivals, telling FOX News, "This can only encourage the extremist elements in the country and the region."

Polls show McCain statistically tied for first place in New Hampshire, which is only days away from holding its Jan. 8 primary, and the Pakistan attack gave McCain a platform to highlight his foreign policy knowledge. McCain, a Vietnam War veteran, has bluntly attacked his rivals for months, saying they are ill-equipped compared with his experience.

Speaking to supporters in Des Moines, Iowa, McCain said he knew Bhutto personally, and knows Musharraf, and that "I know how to best address this situation."

In a statement, McCain said: "The death of Benazir Bhutto underscores yet again the grave dangers we face in the world today and particularly in countries like Pakistan, where the forces of moderation are arrayed in a fierce battle against those who embrace violent Islamic extremism. Given Pakistan's strategic location, the international terrorist groups that operate from its soil, and its nuclear arsenal, the future of that country has deep implications for the security of the United States and its allies. America must stand on the right side of this ongoing struggle."

Former Tennessee Sen. Fred Thompson told FOX News that he too has met Musharraf, and urged the country to crack down on terrorism.

"This is part of a bigger problem ... this is not a part of a criminal investigation anymore where we find the bad guys and bring them to justice. It's a war," Thompson said. "We're going to have to walk that line between democracy on one hand and stability on the other."

In Iowa, where the most recent presidential debate centered on the economy and federal spending, the Bhutto assassination altered the tone of the race in the final stretch, redirecting voters to the issue of extremism and America's role overseas. Candidates who have hammered the issue of foreign policy were given the window to restate their cases. Others who have been accused of lacking international know-how began to toughen their rhetoric Thursday.

"I think it is an opportunity for the candidates who have made (experience) a hallmark of their campaigns -- John McCain, Rudy Giuliani -- and it does present challenges for candidates like Barack Obama and Mike Huckabee," GOP strategist Andrea Tantaros told FOX News.

For Huckabee, the former Arkansas governor who has rocketed to the top of the polls in Iowa, foreign policy experience has been a weakness -- one that he has even joked about.

His statement Thursday on Bhutto initially steered away from aggressive calls to fight terrorism. But he later posted an entry on his campaign Web site saying: "Benazir Bhutto's assassination should also stand as a stark reminder for those who doubt the nature of our enemy. I believe that we are currently engaged in a world war. Radical Islamic fascists have declared war on our country and our way of life ... We must fight the war on terror with the intensity and single-mindedness that it deserves." 

Asked at a stop in Orlando, Fla., how he would respond to the situation as president, Huckabee said, "The first thing I think we would do is try to make sure we monitor closely who is behind it, who is responsible."

He also offered his "sincere concerns and apologies" on behalf of Americans for the incident. He declined to comment on Musharraf's policies toward Islamic extremism, saying it was not the appropriate time for such judgments.

Texas Rep. Ron Paul, though, diverged from the message of other Republican candidates, calling on the United States to cut off aid to Pakistan.

"We should mind our own business, and stay out of supporting military dictators, and that's what Musharraf is," Paul told FOX News. "We're right in the middle of this, and we just sort of stir the pot, and now I'm scared to death we're going to be marching in there and have another war."

Among the Democratic candidates, Hillary Clinton's reaction was personal. The two knew each other, and Bhutto was Clinton's host when she visited Pakistan in 1995.

"I knew her as a leader,"Clinton said in Lawton, Iowa. "I knew her as someone who was willing to take risks to pursue democracy on behalf of the people of Pakistan."

In a world of such violence and threats, Clinton said, "It certainly raises the stakes high for what we expect from our next president. I know from a lifetime of working to make change."

Clinton then directed the focus toward Iowa, drawing comparisons between democracy in Pakistan and the United States.

"She has given her life for that hope (of democracy), and I know that the people of our country stand in solidarity with those that believe as we do in the rights of people to be heard at the ballot box," she said. "It's a particularly poignant moment for us. ... We take our elections seriously. We know how important it is, the decision that will start here in Iowa in one week, picking a president who is ready on Day One to deal with the myriad of problems." 

Clinton has hammered opponent Barack Obama throughout the campaign for being light on foreign policy experience. She has called him inexperienced and naive on that front.

Obama attempted to take a firm tone Thursday in Des Moines, Iowa, expressing sympathy to the people of Pakistan before delving into his stump speech.

"She was a respected and resilient advocate for democracy for the people of Pakistan. We mourn her loss," Obama said. "Our thoughts and prayers are with her family and her supporters, and we want to make clear that we stand with the people of Pakistan in their quest for democracy and against the terrorists who threaten the common security of the world."

Obama then returned to a campaign theme, stressing that he's the only candidate who can bring true change in Washington, and making his case against Clinton without naming her.

The two campaigns got in a minor spat over the political treatment of Bhutto's death, after Obama adviser David Axelrod reportedly said Clinton would have to defend her initial support for the war in Iraq because it diverted resources from fighting Al Qaeda in Afghanistan, and that they may have been involved in Thursday's attack.

"This is a time to be focused on the tragedy of the situation, its implications for the U.S. and the world, and to be concerned for the people of Pakistan and the country's stability," Clinton spokesman Jay Carson said in response. "No one should be politicizing this situation with baseless allegations."

Other Democratic candidates offered specific guidance for the president Thursday. 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.

Delaware Sen. Joe Biden urged Pakistani leaders to open an investigation and said in a statement that the United States should offer any needed assistance, including investigative teams.

Bhutto was shot in the neck and chest before a bomber blew himself up at a campaign rally. At least 20 others also died. The assassin struck just minutes after Bhutto addressed a rally of supporters in Rawalpindi, Pakistan.

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

FOX News' Carl Cameron, Major Garrett, Mosheh Oinounou, Cristina Corbin, Shushannah Walshe and Aaron Bruns and The Associated Press contributed to this report.