The months-long battle over a proposed U.S.-Mexico border wall is set to escalate dramatically when President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE declares a national emergency.

Talk of an emergency declaration immediately sparked division, and in some cases outright rebuke, from the GOP senators Trump will need on his side.

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Senators on both sides of the aisle are steeling themselves for an entrenched, messy fight, with the declaration likely serving as an opening salvo in a high-profile political and legal battle.

Several Republicans panned talk of Trump moving forward with the plan, an option he has kept on the table as conservative allies fume that the deal to fund the government included only $1.375 billion for border barriers, instead of the $5.7 billion for a wall sought by the president.

“I believe it’s a mistake on the president’s part. I don’t believe that the National Emergencies Act contemplated a president repurposing billions of dollars outside of the normal appropriations process,” said Sen. Susan Collins Susan Margaret CollinsSenate 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-Maine), characterizing Trump's decision as "of dubious constitutionality."

Sen. Lisa Murkowski Lisa Ann MurkowskiSenate GOP aims to confirm Trump court pick by Oct. 29: report Senate GOP sees early Supreme Court vote as political booster shot Pence defends Trump's 'obligation' to nominate new Supreme Court justice MORE (R-Alaska) added that she didn’t “think that this is a matter that should be declared a national emergency.”

Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.), who has worked to cultivate a friendship with Trump, also characterized himself as “not in favor of operating government through emergency,” and insinuated that the move could violate the Constitution.

Trump has long floated that he might declare a national emergency to construct a U.S.-Mexico border wall if Congress wasn’t able to come up with a funding deal, despite pushback from top leaders on Capitol Hill.

The White House was reportedly exploring other ways Trump could try to circumvent Congress to get more money without declaring a national emergency.

But top GOP allies on Capitol Hill warned that would not get Trump to $5.7 billion, and signaled they believed he would ultimately carry out the controversial move.

Rep. Mark Meadows Mark Randall MeadowsHouse moves toward spending vote after bipartisan talks House Democrats mull delay on spending bill vote Southwest Airlines, unions call for six-month extension of government aid MORE (R-N.C.), who is close with Trump, said Wednesday that it “would be political suicide” for Trump to sign the funding bill without taking executive action to reroute funds toward the wall.

“I think there's very little political liability from conservatives,” Meadows said about Trump coupling his signature of the bill with executive action.

In a boost for the president, Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.) announced on the Senate floor he would support Trump’s decision, a political U-turn from weeks ago when McConnell warned against an emergency declaration.

“I’m for whatever works, which means avoiding a shutdown and avoiding the president feeling he should declare a national emergency,” he said two weeks ago during a press conference with reporters.

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Many congressional Republicans said they were taking a wait-and-see approach, and that their support or opposition would depend on the specifics of Trump’s declaration.

Sen. John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (R-Texas), who is close to McConnell, said he wanted to see what form Trump’s action took but noted that he previously raised concerns about executive action.

“My concerns about an emergency declaration were the precedent that’s going to be established,” Cornyn said. “I also thought it would not be a practical solution because there will be a lawsuit filed immediately.”

Cornyn noted that McConnell’s floor announcement came after hours of back-and-forth talks between the GOP leader's office and the White House about whether the president would sign the funding bill and under what circumstances.

Sen. Ron Johnson Ronald (Ron) Harold Johnson CIA found Putin 'probably directing' campaign against Biden: report This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report MORE (R-Wis.) also shrugged off McConnell’s decision to support the emergency declaration, saying, “That’s obviously one of the things the leader believes he has to do to get the president to support” the funding bill.

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.) said his stance will depended on the “structure” of the declaration, but added that he generally doesn’t “think that’s a good approach, but we’ll have to deal with it.”

Sen. 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.), who chairs the Senate Appropriations Committee and was among the four legislators who finalized the funding deal, gave full-throated support to the move.

“I’m not concerned because I think the president is on the right track to secure the border, which I share. I like what he’s doing,” Shelby said, adding that presidents going back to Franklin Delano Roosevelt have expanded the power of the executive.

The decision for Trump to declare a national emergency left lawmakers in a familiar pattern: Being asked to react to something the president will do while largely in the dark themselves about the details.

And news of the president’s plans came at an already chaotic moment in the Capitol, as police officers were escorting senators through the basement to elevators that would take them to the Senate floor to vote on the funding deal.

“I’m going to wait and see exactly how he does it and what he does,” said Sen. Roy Blunt Roy Dean BluntSocial media platforms put muscle into National Voter Registration Day Senate GOP faces pivotal moment on pick for Supreme Court This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda MORE (R-Mo.), an appropriator. “There are ways you could transfer funds that I could be fully supportive of, and there are other ways that I have a lot of problems with.”

Democrats are likely to challenge Trump’s executive order in Congress, in addition to legal challenges in the courts. Though rare, lawmakers believe they could pass a resolution of disapproval by a simple majority; it would be subject to a veto.

“He’s trying an end-run around Congress in a desperate attempt to put taxpayers on the hook for it. The Congress will defend our constitutional authorities,” House Speaker Nancy Pelosi Nancy PelosiPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare House lawmakers reach deal to avert shutdown Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill MORE (D-Calif.) and Senate Minority Leader Charles Schumer Chuck SchumerSenate Democrats introduce legislation to probe politicization of pandemic response Schumer interrupted during live briefing by heckler: 'Stop lying to the people' Jacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee MORE (D-N.Y.) said in a joint statement Thursday.

To get a resolution blocking the action through the GOP-controlled Senate, Democrats would need to remain united and flip at least four Republicans.

They excoriated the move on Thursday, warning Republicans that supporting Trump now could haunt them down the line.

"A Democratic president can declare emergencies, as well," Pelosi said Thursday. "So the precedent that the president is setting here is something that should be met with great unease and dismay by the Republicans."

Sen. Chris Van Hollen Christopher (Chris) Van HollenCongress must finish work on popular conservation bill before time runs out Democrats fear Russia interference could spoil bid to retake Senate Mid-Atlantic states sue EPA over Chesapeake Bay pollution MORE (D-Md.), an appropriator, said a declaration would be “a gross abuse of power” and that it was “likely illegal.”

Sen. Jon Tester Jonathan (Jon) TesterPence seeks to boost Daines in critical Montana Senate race This World Suicide Prevention Day, let's recommit to protecting the lives of our veterans Filibuster fight looms if Democrats retake Senate MORE (D-Mont.), a moderate from a Trump-won state, said the decision was “not smart at all.”

“I think it sets a standard for declarations of emergency that just about anything could fit into, and he isn’t going to be the president forever,” Tester said. “I think it takes power away from the legislative branch, so it’s a failure on all sorts of fronts.”