Stymied by Congress, President Donald Trump has turned to the gargantuan Pentagon budget as a possible source of funding for a wall along the U.S.-Mexican border, according to an exclusive report by Just Security.

Kate Brannen, deputy managing editor of the publication and a nonresident fellow at the Brent Scowcroft Center on International Security, said Friday on “The Laura Ingraham Show” that most of the money for defense is off-limits. But there is some “wiggle room,” she said.

Secretary of Defense James Mattis is not a fan of the idea.

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“But he does feel like he has to present the president options,” Brannen said. “That’s his job.”

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The $1.3 trillion spending plan that the president reluctantly signed last week includes about $1.6 billion for border security, but most of it is earmarked for repairing existing fencing and infrastructure.

That prompted Trump on Sunday to tweet that America should “Build WALL through M!” — meaning the military.

Brannen said there are some options, but she added there is not a lot of flexibility in spending defense funds for purposes not specifically authorized by Congress.

“It is really limited, and it risks sort of angering folks over on Capitol Hill, especially on the defense committees, if they attempt to do that,” she said.

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For instance, she said, the Pentagon has $500 million for a counter-narcotics program. The Defense Department also has a pair of emergency construction funds that theoretically could be redirected toward the wall.

One fund has $50 million devoted to last-minute construction projects tied to defense. Another fund, which has no budgetary cap, can be used during a declaration of war or a national emergency.

Brannen said there are several large political and legal problems weighing against such creative financing.The Pentagon could argue that building the wall serves the anti-drug purpose of the counter-narcotics program, Brannen said. But she added it simply does not provide much money.

Because of the $700 & $716 Billion Dollars gotten to rebuild our Military, many jobs are created and our Military is again rich. Building a great Border Wall, with drugs (poison) and enemy combatants pouring into our Country, is all about National Defense. Build WALL through M! — Donald J. Trump (@realDonaldTrump) March 25, 2018

“It’s a drop in the bucket, and you’re taking away from actual counter-narcotics programs that, you know, are working in South America to do sort of this same thing,” she said.

The emergency construction funds are more promising, but Brannen noted that a declaration of war is not likely and added that declaring illegal immigration an emergency would face strong resistance.

“A national emergency is possible, but that case would have to be made. Why is this all of a sudden an emergency?” she said.

Brannen said tapping construction funds also would divert money from the Defense Department’s regular projects, “which lawmakers are hugely protective of because that’s how they bring jobs and dollars back home to their districts. So that’s going to cause a fight on Capitol Hill.”

Brannen pointed out one other possibility to beef up border security that would not involve new construction — the National Guard.

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“There are some complications with that, depending on who mobilizes them, the federal government or the state or the governors, in terms of what they can do,” she said.

Brannen said the most likely scenario would be a mobilization in which the state calls out the Guard and the federal government picks up the tab.

Bottom line, Brannen said, there is no great substitute for Congress earmarking billions of dollars toward wall construction.

“For a project this big, you have to get Congress to sign onto it, to appropriate the money specifically for it,” she said. “Otherwise, you’re kind of grasping at straws. Even though the defense budget is as big as it is, it has, you know, it has very specific funding lines that it’s supposed to go towards.”

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.