Republican lawmakers are skeptical of a plan floated by President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE to have the military build a wall along the U.S.-Mexico border or repurpose defense funds to pay for it.

Trump on Tuesday tweeted that he would order military personnel to complete construction of border fencing and barriers if Democratic leaders refuse his request to provide $5 billion for construction of a border wall.

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“If the Democrats do not give us the votes to secure our Country, the Military will build the remaining sections of the Wall. They know how important it is!” Trump tweeted Tuesday morning.

Trump had previously floated in private conversations with GOP lawmakers the idea of repurposing defense funding to help build the wall, but the idea was greeted with skepticism, according to Senate GOP sources.

“There was a discussion of it, but on the question of how to work it there are a lot of closed doors,” said a GOP lawmaker familiar with internal discussions about repurposing defense funding to build the wall.

GOP lawmakers worry that agreeing with Trump’s plan would cede significant spending authority to the executive branch and set a bad precedent. They are also concerned about how it might impact defense spending priorities.

“What would [the Defense Department] give up to pay for that?” said the GOP lawmaker. “That’s the question.”

Sen. Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (R-Fla.), a member of the Senate Appropriations Committee, said there is legal uncertainty about whether Trump could repurpose defense funding.

“If the president has the authority to do it legally, if he has the authority to do it, I would think politically it’s one of his key campaign promises and he would want to keep it. But I don’t know if he has the legal authority and I’m not sure what kind of friction that would create with appropriators around here in future negotiations on other topics.”

Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderGraham: GOP has votes to confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda MORE (R-Tenn.), a member of the Defense Appropriations Subcommittee, said he wants to wait to see if Trump’s idea has any legs.

“I will wait until the idea moves from the level of a tweet to a specific proposal and then I’ll think about it,” he said.

Sen. Lindsey Graham Lindsey Olin GrahamSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-S.C.), one of Trump’s staunchest allies in the Senate, predicted there would be “institutional pushback” if the president attempts to reprogram defense money for borders security.

“It’s better to go through Congress,” he said. “We repurpose money all the time but you have to get it approved, generally speaking. I think there’ll be an institutional pushback against a unilateral repurposing."

“It’s not about Trump,” Graham, a member of the Senate Appropriations Committee, added. “I think there will be some concern — it’s got nothing to do with the wall, I clearly support the wall — is whether or not the body would feel comfortable with that.”

Senate Appropriations Committee Chairman Richard Shelby Richard Craig ShelbySenate GOP eyes early exit Dems discussing government funding bill into February GOP short of votes on Trump's controversial Fed pick MORE (R-Ala.) said Trump “has under the Constitution, as commander in chief, a duty to defend, to protect the borders.”

Asked if Trump had the power to reprogram funding, Shelby replied, “depends what he wanted to do.”

“He has some latitude but I don’t know how much latitude,” he added.

Updated: 12:33 p.m.