President Donald Trump could declare a national emergency to build his border wall — but federal law may not give him carte blanche to tap billions of dollars from the military budget.

Trump is set to address the nation on immigration from the Oval Office at 9 p.m. Tuesday, after days of speculation that he will invoke the National Emergencies Act to end the impasse with congressional Democrats that has triggered an 18-day partial government shutdown.

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The fate of such an effort could depend on how courts interpret specific language in federal statutes governing use of the armed forces.

A formal declaration of a national emergency would allow some military construction funds to support “such use of the armed forces” as the crisis requires, according to Section 2808 of Title 10. After former President George W. Bush invoked the Emergencies Act following the attacks of Sept. 11, 2001, the military used that authority for construction projects in Iraq and Afghanistan as well as to improve counterterrorism security on some stateside bases.

But the Trump administration would still have to use funds that the Pentagon has already set aside for its own construction projects. And it would have to make the case that the emergency requires the military’s involvement.

That’s a subjective question that courts will challenge, said Sam Berger, a former senior counselor at OMB who is now with the Center for American Progress.

“The president can declare an emergency, but to use these military authorities, he has to declare an emergency that requires the use of the military,” Berger said. “Trump is trying to flip this on its head. He’s saying he wants to use the military to support the building of the wall, not that building the wall will support the military. That’s not what Congress intended and it’s not what the law says.”

Still, the legal questions could cut both ways, said Margaret Taylor, a senior editor at the national security law site Lawfare.

On the one hand, “a judge could very well say, ‘I’m skeptical that there’s a really national emergency and I’m halting construction until these issues are sorted out,’” said Taylor, who worked for the Senate Foreign Relations Committee until last summer.

But on the other hand, the administration’s deployment of active-duty troops to the Southern border last fall could “lay the groundwork” for an argument that the wall is being built to support their operations, fulfilling the letter of the law, Taylor said.

“Those troops were deployed for political reasons and this is being done for political reasons, too, but having them there could allow the White House to be a little bit more convincing about those legal arguments,” Taylor said. “I don’t know if this was intentional when the troops were deployed, or if the administration is just taking advantage of it now.”

Although Trump called publicly for as many as 15,000 active-duty troops to go to the border, the deployment peaked at around 5,900 troops and was down to some 2,300 troops last week. Many of the troops deployed were engineers who spent their time on the border placing concertina wire, concrete barriers and other fortifications around ports of entry.

Those troops are formally deployed in support of civilian law enforcement authorities, however, as are 2,100 National Guard troops mobilized last spring.

It’s unclear whether uniformed construction engineers would be sent back to the border to build the wall if it were funded with military money, or whether the military construction funds would be used to pay civilian contractors to build the barriers.

The prospect of Pentagon funds being used to build the wall “creates a lot of practical concerns in addition to the obvious legal issues,” Berger said.

Section 2808 would allow the use of “unobligated” military construction funds not yet committed to other contracts, of which the Pentagon has about $10 billion in this fiscal year’s budget and another $13 billion from past budgets, according to The Washington Post. That’s enough to cover both the $5.7 billion that Trump wants in a funding bill and tens of additional billions that a wall would probably require.

But even those “unobligated” funds have largely already been approved by Congress for use in other military construction projects.

“You’d have to divert funding from projects the military has already decided it needs, like building schools on military bases and improving base security, and that’s going to be upsetting both for the military and for the members of Congress who approved those projects,” Berger said.

Further complicating matters, Section 2808 authorizes construction only on military land — very little of which abuts the border.

“The president would need to use the military to seize hundreds upon hundreds of miles of border land from the states and from private citizens, which there’s no legal authority to and which you can bet would create a political and legal response,” Berger said.

It’s unclear what Trump meant when he said last week that he would use “the military version of eminent domain” for such seizures, Taylor said.

Besides Section 2808, most other authorities that would allow military construction in a national emergency are ill-suited for Trump’s purposes, Taylor said. One related authority caps out at just $50 million and cannot be used for projects, like the wall, for which Congress has previously denied military construction funding. And a provision for building barriers for counter-narcotics purposes would require arguing that the wall is a “minor military construction project,” an argument unlikely to hold water given the proposed costs.

But another legal authority, Section 2293 of Title 33, allows the Army to divert funds from civil works projects during a national emergency. That could allow wall construction to be funded not from the usual military construction funds but from civil works projects run by the Army Corps of Engineers.

In either case, though, much depends on how lawmakers and judges react to whatever emergency funding mechanism Trump announces — and how quickly.

“How deferential will Republicans on the Hill be and how deferential will lawmakers be?” Taylor asked. “Our Constitution is structured so that Congress makes the laws and the president can’t just go around them and do it himself, which the circumstances make it seem like is what’s happening here.”

2020 elections

Two Republican senators have already broken with Donald Trump over his demand for billions in border wall money to reopen the government.

Republicans face a tooth-and-nail struggle to keep their Senate majority in 2020 under the best of circumstances. But the prolonged government shutdown has gotten them off to a rough start, exploiting party divisions on immigration and giving Democrats fodder to attack Republican incumbents out of the gate.

Though the shutdown itself is likely to be long forgotten by November 2020, it’s already laid bare the schism incumbent senators will have to navigate — between President Donald Trump’s hard-line immigration backers and independent voters whom polling shows oppose the shutdown and the president’s border wall.

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Two of the most vulnerable Republicans on the ballot next year, Sens. Cory Gardner of Colorado and Susan Collins of Maine, have already broken with Trump and called for the government to reopen even without an agreement on the border wall. Other Republican senators haven’t gone there yet, but are growing increasingly frustrated by the impasse.

“It hurts all of us and everybody that’s looking from the outside, they’re like: What is wrong with you? Why can’t you find a solution?” said Sen. Joni Ernst of Iowa, one of the Republicans up for reelection next year.

Another, Sen. Shelley Moore Capito of West Virginia, who chairs the appropriations subcommittee that funds the Department of Homeland Security, said last week she is still pushing the president to support her legislation that would fund border security at a lower amount than the president wants. It passed her committee last year with bipartisan support.

“A shutdown in my view is a no-win proposition,” said Capito, who is seen as a relatively safe bet to keep her seat. “When I was going through TSA this morning coming up here [to Washington], my local TSA guys were like ‘Are we going to get paid?’ It’s just a lot of unneeded stress on a lot of people.”

House Democrats are using their new power in the majority to amp up the pressure on Senate Republicans. Last week, they voted to end the shutdown by passing a package that would reopen the government but only temporarily fund the Department of Homeland Security.

Trump threatened to veto the proposal and Senate Majority Leader Mitch McConnell (R-Ky.) refused to bring the legislation up for a vote, arguing it was fruitless to vote on anything that Trump wouldn’t sign. House Democrats plan to continue to force the issue by passing individual appropriations bills this week.

Collins and Gardner, both of whom are from states Trump lost in 2016, called for different solutions to end the shutdown without a border wall. Collins said she supported passing individual appropriations bills unrelated to the wall debate.

“I’m not saying their whole plan is a valid plan, but I see no reason why the bills that are ready to go and on which we’ve achieved an agreement should be held hostage to this debate over border security,” Collins said.

Gardner told The Hill last week that Congress should pass a continuing resolution to temporarily fund the government, which the Senate passed by voice vote in December before House Republicans and the president rejected it. Colorado is an increasingly blue state with a large and growing Latino population, making immigration one of the thorniest issues for the first-term Gardner.

Polling has shown Trump taking the brunt of public blame for the shutdown. A Reuters poll this week showed more than half of respondents pinning it on the president, while one-third pointed the finger at congressional Democrats. Nearly half of respondents said they oppose funding for a border wall with Mexico.

A POLITICO/Morning Consult poll in December showed more than half of independent voters said a border wall was not important enough to shut down the government over.

Other Republicans have stuck together on the negotiations, roundly rejecting proposals to fund the government without a down payment on the wall. Sen. Thom Tillis (R-N.C.), who is likely to be one of Democrats’ top targets next year, and Sen. Lindsey Graham, (R-S.C.), who is running for reelection in a relatively safe red state, have both proposed broader immigration agreements both funding the border wall and securing a solution for young immigrants protected by an Obama-era executive order. A spokesman for Tillis said the senator opposes a CR or individual appropriations bills because a solution requires Trump’s signature.

The Senate Democratic campaign arm, meanwhile, is trying to make GOP senators up in 2020 squirm. Senate Republicans “own every miserable consequence of their pointless shutdown,” David Bergstein, a spokesman for the Democratic Senatorial Campaign Committee, said in a statement.

Josh Holmes, a top adviser to McConnell, said it wasn’t surprising that senators are increasingly frustrated by the impasse. But he noted that, for the most part, Republicans have remained united.

“I think there’s a difference between frustration and political concern,” Holmes said. “Clearly anytime you’re in a standoff like this, it’s a frustrating experience.”

Burgess Everett contributed to this report.