Democrats are set to grill the Pentagon about the troop deployment to the U.S.-Mexico border amid what are expected to be contentious negotiations over border security funding to keep the government open.

The House Armed Services Committee’s first hearing of the new Congress, scheduled for Tuesday morning, will see lawmakers question John Rood, under secretary of Defense for policy, and Vice Adm. Michael Gilday, director of operations for the Joint Staff, about the “Department of Defense’s Support to the Southern Border.”

The hearing comes days after the end of a partial government shutdown over President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE’s demands for money for his proposed border wall and the possibility that he will declare a national emergency in three weeks if funding negotiations do not go his way.

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Declaring a national emergency could unlock Pentagon funding to be used for the wall. But Democrats have lambasted that idea as executive overreach and warned it would raid military coffers at a time when readiness is shaky.

“I don’t think there is an emergency even under the 1976 law, so there would immediately be a lawsuit,” House Armed Services Committee Chairman Adam Smith David (Adam) Adam SmithWhen 'Buy American' and common sense collide Overnight Defense: Marine Corps brushes off criticism of Marines' appearance in GOP convention video | US troops injured in collision with Russian vehicle in Syria | Dems ask for probe of Vindman retaliation allegations Democrats press Pentagon watchdog to probe allegations of retaliation against Vindman brothers MORE (D-Wash.) said on C-SPAN’s “Newsmakers,” referring to the National Emergencies Act. “So taking billions of dollars out of the Pentagon’s military construction budget would be a big problem. There’s bipartisan opposition to it.”

U.S. troops were first deployed to the border last year shortly before the midterm elections as Trump fixated on a caravan of asylum seekers who were traveling from Central America.

The deployment peaked at about 5,900 troops and was originally supposed to end Dec. 15. The mission has since been extended twice, first to Jan. 31 and now to Sept. 30.

Right now, there about 2,350 troops at the border.

The military’s task so far has largely been to put up concertina wire in border areas of California, Arizona and Texas the administration said needed fortification.

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With the last extension of the mission, the Pentagon said the focus would shift from “hardening ports of entry to mobile surveillance and detection.”

The Department of Homeland Security request for an extension also asked for more assistance at the border. Pentagon officials have not yet made a decision on whether that assistance will come in the form of more military personnel or equipment.

A Pentagon spokesman told The Hill that decision will come by the end of the week.

Critics of the deployment, including congressional Democrats, accused Trump of stoking fears to rile up his base ahead of the elections and of using the military for political purposes. Since then, the debate over the Pentagon’s role at the border has only become more fraught following the partial government shutdown.

The 35-day shutdown ended Friday when Trump agreed to a continuing resolution through Feb. 15. During those three weeks, a bipartisan, bicameral conference committee will meet to negotiate border security funding.

Trump has demanded $5.7 billion to build his border wall. Democrats, though, have shown no sign of being willing to give him wall funding, and Trump himself has said he sees a “less than 50-50” chance of the negotiations ending in a deal.

Without border wall funding, Trump has threatened to either shut down the government again or declare a national emergency. Such a declaration could allow military construction funding or Army Corps of Engineers civil works funding, including a portion of the $13.7 billion meant for disaster relief, to be used for the wall.

On Monday, White House press secretary Sarah Huckabee Sanders said the lack of a deal would “force” Trump “to take executive action that does not give Democrats the things they want.”

“The president’s No. 1 duty and the No. 1 responsibility he sees as commander in chief is protecting the American people,” she said at a press briefing. “He sees the crisis at the border to be a real one.”

Trump previously floated the idea of declaring an emergency in the middle of the shutdown, but he backed off amid warnings of potential court challenges and opposition to using military or disaster relief funding.

Smith said he hopes to use Tuesday’s hearing to probe outstanding questions on the border deployment, including what work troops are doing now and how much the mission has cost so far.

“The biggest thing we want at this point is transparency,” Smith said on C-SPAN. “What is the purpose of the active-duty troops that are sent there? Because it is a great cost to the Pentagon. These active-duty troops are now not doing the training they need to be prepared to meet the national security threats that are their paramount job.”

Smith also said he has “larger concerns” about the “militarization of the border,” explaining he wants to ensure the military is not being used for domestic law enforcement as prohibited by U.S. law.

Dozens of Democrats are also pushing Smith to use the annual defense policy bill to block Pentagon funding for the wall. Fifty-one House Democrats, including some Armed Services members, sent Smith a letter last week asking him to work to prevent the military from “becoming a pawn in Trump’s immoral, wasteful, and potentially illegal border ‘wall.’ ”

On the Republican side of the dais, ranking member Rep. Mac Thornberry William (Mac) McClellan ThornberryTrump payroll-tax deferral for federal workers sparks backlash Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq Top Armed Services Republican 'dismayed' at Trump comments on military leaders MORE (R-Texas), while supportive in general of a physical barrier at the border, has expressed concern about using military construction funding for a wall, saying he has seen firsthand deteriorating bases that need the money.

Outside experts who have studied military readiness are similarly skeptical.

Eric Edelman, a former ambassador and under secretary of Defense who co-chaired the National Defense Strategy Commission, said Monday he agrees with Thornberry. The commission recently warned of “grave and lasting” consequences if Washington does not adequately fund the Pentagon.

“It would be inappropriate to declare a national emergency to use the defense budget as a piggy bank in which to fix this immigration dispute between the Pentagon and the Congress,” Edelman said at the Foundation for Defense of Democracies in response to a question from The Hill.

Still, some of Trump’s close allies are continuing to press for an emergency declaration in the event that Congress is unable to reach a deal on border security over the next three weeks.

Sen. Lindsey Graham Lindsey Olin GrahamMomentum growing among Republicans for Supreme Court vote before Election Day Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral Warning signs flash for Lindsey Graham in South Carolina MORE (R-S.C.), a member of the Senate Armed Services Committee and one of the most outspoken lawmakers in advocating for Trump to use his emergency powers for a wall, said Monday that the president should go ahead and declare a national emergency if no deal is reached.

Graham wrote on Twitter that a legislative deal would be “the best way forward,” but directing troops to build a wall would also be consistent with how past administrations have used troops at the border.

“Presidents Bush, Obama, and Trump have all sent troops to help secure the border in the past,” he tweeted. “What’s the difference between troops securing the border and troops constructing barriers to secure the border?”