In front of a national TV audience Tuesday night, Gov. Christie kept returning to one phrase to promote his presidential credentials: "As a former federal prosecutor ... ."

The second-term New Jersey governor, who rose to national prominence by battling unions and responding to a natural disaster, has increasingly been promoting his previous job on the campaign trail. He has opened recent events by focusing on national security, noting his experience as U.S. attorney for New Jersey after the Sept. 11 attacks and touting terror prosecutions he led.

The new emphasis has come as the focus of the presidential race has turned to foreign policy following the recent terror attacks in Paris and San Bernardino, Calif.

Christie, who spoke of his background as a federal prosecutor this spring while arguing for greater government intelligence-gathering capabilities, is asserting that his experience makes him uniquely qualified to handle what he calls the president's key duty.

"For seven years, I spent my life protecting our country against another one of those attacks," Christie said at the end of Tuesday's GOP presidential debate, referring to 2001. "You won't have to worry when I'm president of the United States whether that can be done, because I have already done it."

In playing up his resumé as U.S. attorney - a position that paved the way for his 2009 gubernatorial election, as he gained a reputation as a corruption-buster - Christie is not only trying to capitalize on the shift toward terrorism as a primary issue in the campaign, but also drawing a contrast with Republican front-runner Donald Trump, said Julian Zelizer, professor of history and public affairs at Princeton University.

"He's saying, 'I've actually worked on this, I understand what it means.' Implicit is, Donald Trump doesn't," Zelizer said.

Focusing on his prosecutor credentials has another benefit: shifting discussion "away from New Jersey, which isn't always favorable," Zelizer said, noting the George Washington Bridge scandal and the state's lagging economic recovery.

But he said Christie wouldn't downplay his credentials as governor - a resumé point presidential candidates like him often use as a "chief weapon" to cast themselves as more equipped to assume the presidency than legislators.

Christie made that case also Tuesday, accusing his rivals from the Senate of acting like "bystanders" instead of taking responsibility for cuts to the military. "This is the difference between being a governor and being in the legislature," he said. "When something doesn't work in New Jersey, they look at me, say, 'Why didn't you get it done?' "

At recent campaign events, Christie has responded to questions about various issues by touting aspects of his record as governor. Asked in Iowa recently about Planned Parenthood, Christie said he'd "ended Planned Parenthood funding in my first year as governor."

To a man who asked how he would address "deteriorating" race relations, Christie spoke of community policing in Camden, telling voters that "I fired the entire police department in Camden, N.J." The replacement of the city police with the current county force was carried out at the local level, though Christie supported it.

But lately he has been casting his role as U.S. attorney as the experience that sets him apart.

Describing how he would respond to a briefing by FBI Director James Comey, Christie said at a town-hall meeting this month in Mason City, Iowa: "He's not going to have to say, 'Do you understand these acronyms that I'm using, Mr. President?' Because I've used them. He's not going to say, 'Do you know what actionable intelligence is, Mr. President?' Because he knows I do."

At an earlier event during that trip, Christie described two terrorism cases brought by his office. "What I fear now is, we're forgetting the Fort Dix Six. We're forgetting Hemant Lakhani," he said, addressing a meeting of rural electrical cooperatives.

Those cases, which Christie has often cited while calling for heightened intelligence, were "important cases from the point of view of getting a lot of attention and getting high sentences," said Karen Greenberg, director of the Center on National Security at the Fordham University School of Law.

"But in terms of actual threat to the country," Fort Dix and Lakhani "weren't at the level" of cases that involved interrupted attacks, Greenberg said. Both cases involved government informants "seeing just how far these guys would go," Greenberg said. "That's a different thing than interrupting a crime."

Three of the five men convicted in the Fort Dix case - the Duka brothers - have been granted a court hearing regarding a claim that they were improperly compelled by defense attorneys not to testify.

Christie backers describe his background as U.S. attorney as an added benefit in a race featuring several other governors, but not the sole reason for supporting him.

"He had more experience than the other governors we looked at," said Joe McQuaid, publisher of the Manchester (N.H.) Union Leader, which recently endorsed Christie. "Part of the reason for the more experience" was Christie's prosecutor role, he said.

Donna Sytek, a former New Hampshire House speaker, arrived at the decision to endorse Christie in part because of his background as a prosecutor - but not related to terrorism cases.

"I had been concerned about him being from New Jersey. New Jersey has a reputation," said Sytek, who said she decided to support Christie before the attacks last month in Paris. "Don't they all have skeletons in their closets?"

But she was "impressed" that Christie had prosecuted 130 corruption cases. "That, and his executive experience, gave me a second look," she said.

John H. Sununu, a former New Hampshire governor who later served as chief of staff to President George H.W. Bush, said Christie has the right experience to be president, but isn't the only Republican running who does.

"I'm comfortable with the Christies, the Bushes, the Kasichs, the Rubios," Sununu said, describing his preference for governors who have made policy decisions "in the public eye." He included Marco Rubio in that group based on the Florida senator's role as a former House speaker in his state, "a demanding job" that would have taught him how to deal with a legislature, Sununu said.

While he described governing experience as key, other jobs "add to capacity," Sununu said. He said that "for people to think anyone who just has business experience can go in and be president is a huge mistake."

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