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Several senior Senate Republicans and members of GOP leadership raised serious concerns Monday about the possibility of President Donald Trump bypassing Congress and using an emergency declaration to build a border wall with Mexico.

Those GOP lawmakers said such a move could have broad constitutional problems, practical political implications for Republicans and could set a dangerous precedent for all future presidents who might use the power to advance their policy priorities despite Congress.

The words of caution come as Trump’s demand for billions to build a border wall remains a key point of contention in private negotiations to avert a government shutdown in just 12 days, sources from both parties said Monday.

“There are lot of reservations in the conference about that and I hope they don’t go down that path,” Senate Majority Whip John Thune of South Dakota told CNN about the emergency declaration that Trump has repeatedly said could be his backstop position if talks don’t yield funding for a wall.

Sen. John Cornyn, a Texas Republican, called it a “dangerous step” and “not a good strategy” and that Republicans have conveyed these doubts to the White House “many times. Publicly and privately.”

Cornyn also warned that House Speaker Nancy Pelosi would “introduce a resolution of disapproval that will pass the House and then come over here and divide Republicans.”

Cornyn also warned of the precedent it would set citing several possible Democratic presidential candidates like Sen. Elizabeth Warren of Massachusetts or independent Sen. Bernie Sanders of Vermont.

“The whole idea that presidents — whether it’s President Trump, President Warren or President Sanders — can declare an emergency and somehow usurp the separation of powers and get into the business of appropriating money for specific projects without Congress being involved, is a serious constitutional question,” Cornyn said.

Sen. Roger Wicker, a Mississippi Republican, said, “There’s much concern among people who wish Trump well on this about the slippery slope.”

Sen. Susan Collins of Maine, a Republican who’s up for re-election in 2020, said such a move would be “of dubious constitutionality.”

House and Senate negotiators over the last few days have moved closer to ironing out their differences on measures in the spending legislation that are not tied to border security and barriers sought by Trump, the sources said. They expect the focus of the talks to shift toward Trump’s wall demands and border security after the President’s Tuesday State of the Union speech and once the lawmakers are briefed by experts on the situation at the border later this week.

But time is running short given that lawmakers want to reach a deal by week’s end in order to give time for the House and Senate to jump through its procedural hoops and pass legislation before another punishing shutdown that would affect one-quarter of the federal government.

When asked by CNN if a shut down is off the table, Senate Republican leader Mitch McConnell said “We don’t know quite yet how this is going to end. But I’m hopeful and optimistic.”

Sen. Richard Shelby, an Alabama Republican who’s the chairman of the Senate Appropriations Committee, suggested talks took a step backward at the end of last week when House Speaker Nancy Pelosi, a California Democrat, declared at a news conference, “There’s not going to be any wall money in the legislation.”

“If she says there’s not going to be any, I don’t know where the negotiations go,” Shelby said in response to her hard line.

His concerns are echoed by other Republicans close to the negotiations, who described Pelosi’s acceptance last week of additional vehicle fencing along the border as something that’s already allowed under current law and therefore not a concession from Democrats.

But Trump has also confused negotiators by not clarifying his demands — and it’s uncertain whether his State of the Union address will help or hurt Congress’ ability to cut a deal in just days.

“Listen closely to the State of the Union,” Trump said Friday when asked if he was ready to announce a national emergency to build a wall, should congressional talks not result in a deal he wants.

Meanwhile, McConnell said last week he hopes Trump doesn’t go the emergency route.

Congressional Republicans privately worry that Democrats would force a vote to overturn that emergency declaration, something that would likely pass the Democratic-controlled House and may clear the GOP-led Senate if just a handful of Republicans join Democrats. The issue could divide Republicans and be an ugly battle even if opponents of the wall can’t get enough votes to override a presidential veto.

Shelby told CNN Sunday he would invite Border Patrol officials to Capitol Hill this week to inform negotiators of their needs.

“It’s a question: How do we get off the politics and to the substance? If we can do that, we will get the job done,” Shelby told CNN’s Jake Tapper.

In addition, the four GOP House members who are on the conference committee were on the US-Mexico border Sunday and Monday “to see the situation at the border first-hand and hear directly from experts on the ground” as “they work to come up with a compromise to fund border security,” according to a tweet from the House Appropriations Committee Republicans.