WASHINGTON — Two senior national security surrogates for the presidential candidates traded barbs in a lively debate before a group of military reporters and editors Thursday night. In the end, though, all roads led back to the economy.

Speaking on behalf of Republican nominee Mitt Romney, Dov Zakheim, the under secretary of defense in President George W. Bush’s administration, talked about spending more on defense in terms of the economy by growing the Gross Domestic Product.

“The governor underpins his notion of peace through strength,” Zakheim told a crowd gathered in Washington for the annual Military Reporters and Editors conference. “If you want to have peace and stability, you need to be strong to dissuade people from doing things they might otherwise do.”

Candidate Romney has said that, if elected president, he would make it a goal to spend 4 percent of GDP on defense annually.

But Michele Flournoy, a foreign policy adviser to President Barack Obama’s campaign and a former top defense official in his administration, said Romney would give the Pentagon $2 trillion it isn’t even asking for, echoing Obama’s remarks at the second presidential debate.

Staking out positions likely to come up again on Monday in the third and final presidential debate, where the topic is foreign policy, budget constraints were clearly top of mind for the two senior advisers.

“Peace through strength is a great slogan, but it’s not a foreign policy,” said Flournoy, pushing back on the Romney campaign’s talking point. “Taking defense spending back to 4 percent GDP, what’s the strategy behind that? And how in the world are you going to pay for it?”

The Obama administration has plans to slash ground troop levels by 100,000 military personnel, bringing the number down to about where it was in 2001 before the surge from wars in Iraq and Afghanistan.

“Peace through strength is a great slogan, but it’s not a foreign policy,” countered Flournoy. “Taking defense spending back to 4 percent GDP, what’s the strategy behind that? And how in the world are you going to pay for it?”

Zakheim said that kind of defense spending the Republican ticket advocates could be sustained without raising taxes or cutting major programs in a stronger rebounding economy.

Additional cuts to defense would spread the American military forces too thin as the U.S. shifts focus from the Middle East to the Pacific, perhaps putting the nation at risk of losing its edge in military might, Zakheim said.

“To be credible to your allies, you need that strength. You need that spending,” he said.

Flourney took exception to the idea that Obama doesn’t embrace the notion of American exceptionalism, saying the president does regard America as the world leader but differs from Romney in how to play that role.

“We’re not leading from behind. We’re encouraging others to step up and be part of the solution,” she said.

Both sides seemed to agree on at least one issue — fighting two wars has put an enormous amount of stress on U.S. troops and their families and they need to be cared for.

“Veterans and active-duty service members will not be touched if sequestration goes into effect,” Flournoy said.

“There’s nothing to argue about this,” Zakheim added.