Congress braced Monday for an unprecedented effort to overturn a presidential emergency declaration, as Republicans worked to limit defections on the eve of a critical House vote while Democrats framed the issue as a constitutional showdown.

Partisans on both sides unleashed sharp new rhetoric ahead of Tuesday’s vote on a Democratic-authored resolution that would nullify President Trump’s declaration of a national emergency at the U.S.-Mexico border. Congress has never before sought to cancel a national emergency declared by the president since passage of the National Emergencies Act in 1976.

The resolution is expected to pass the House easily with unified Democratic support. But GOP leaders were urging their members to oppose it, aiming to keep the final tally low enough to demonstrate that Congress would be unable to overturn the veto that Trump has threatened.

While Democrats tried to focus on the constitutional issues at stake in Trump using an emergency declaration to get border-wall money denied by Congress, Republicans trained their arguments on what they called dire conditions along the border that necessitated Trump’s move.

House Minority Leader Kevin McCarthy (R-Calif.) said Monday evening that he wasn’t sure how many Republicans would vote for the resolution, “but there will not be enough to override any veto.”

The outcome was less assured in the Senate, which will be required — under provisions of the National Emergencies Act — to take up the legislation within weeks of House passage. If all Senate Democrats vote for the disapproval resolution, only four Republican votes would be needed to ensure passage, since just a simple majority vote is required.

With numerous Republican senators voicing concerns or outright opposition to Trump’s national emergency declaration, the disapproval resolution was widely expected to pass the Senate — though probably without securing the two-thirds majority that would be needed for a veto override. Senators could also potentially vote to amend the resolution, which could complicate its path to passage.

Trump urged GOP senators to stick with him, writing on Twitter Monday: “I hope our great Republican Senators don’t get led down the path of weak and ineffective Border Security. Without strong Borders, we don’t have a Country — and the voters are on board with us. Be strong and smart, don’t fall into the Democrats ‘trap’ of Open Borders and Crime!”

Democrats contended that Trump was declaring an emergency where none existed, and they argued that he was usurping Congress’ constitutional authority over government spending. They said the real issue at stake in Tuesday’s vote is not Trump’s border wall, but the Constitution and its separation of powers.

“It was a lawless act, a gross abuse of power, and an attempt by the president to distract from the fact that he broke his core promise that Mexico pay for the wall,” Senate Minority Leader Charles E. Schumer (D-N.Y.) said. “There’s no evidence of an emergency at the border.”

House Democratic leaders and rank-and-file lawmakers held a news conference where they displayed a sign that said, “Terminate Trump’s Fake Emergency.”

“I pray that the members of the House will have the conscience and the courage to protect the greatest Constitution the world has ever seen,” said House Majority Leader Steny H. Hoyer (D-Md.). “The American people will be watching.”

During the Obama administration, Republicans focused intently on protecting their constitutional prerogatives as they repeatedly accused President Barack Obama of executive overreach over his unilateral moves aimed at protecting certain groups of immigrants from deportation, among other issues. During those years, House Republicans made retaking power from the executive branch a central plank of their “Better Way” governing agenda, and voted to pass a measure called the Executive Needs to Faithfully Observe and Respect Congressional Enactments of the Law Act of 2014.

Some of these same Republicans argued Monday that the situation is different now, because Trump is relying on a federal law that allows him to declare a national emergency and because, they said, evidence of an emergency was plentiful — even though border apprehensions are at their lowest levels in decades. Republican lawmakers have been reluctant throughout Trump’s presidency to challenge him outright, given the popularity he retains with Republican voters, and his national emergency declaration did not appear to have changed that.

“Does the president have the legal authority to call an emergency, just as 60 times has been done since 1976? The president does have that legal authority to do it,” McCarthy said. “What President Obama did was much different when he changed law in the process. Here what we’re finding is, yes, there is an emergency along the border.”

Sen. Charles E. Grassley (R-Iowa) said that it’s “quite obvious from the Border Patrol, all sorts of information of drugs coming across the border, fentanyl enough to kill 100,000 people, sex trafficking, human trafficking, drugs coming across, criminals coming across. The No. 1 responsibility of the federal government is to protect the American people.”

Asked about concerns that Congress might be ceding some of its prerogatives and creating a precedent that future presidents could use to declare emergencies on a variety of issues, Grassley said the National Emergencies Act should be revisited.

But not all Republicans were ready to support Trump. Sen. Susan Collins (R-Maine) reiterated her intention to vote for the disapproval resolution, and Sen. Lisa Murkowski (R-Alaska) suggested in an interview with an Anchorage television station over the weekend that she would probably do the same.

On Monday, Sen. Thom Tillis (R-N.C.), who is up for reelection in a presidential battleground state, joined them, writing in an opinion piece for The Washington Post: “It is my responsibility to be a steward of the Article I branch, to preserve the separation of powers and to curb the kind of executive overreach that Congress has allowed to fester for the better part of the past century. I stood by that principle during the Obama administration, and I stand by it now.”

Senate Majority Leader Mitch McConnell (R-Ky.) had cautioned Trump beforehand about the possibility that an emergency declaration would result in a vote of disapproval that would force him to issue the first veto of his presidency. But McConnell ultimately agreed to support the declaration as Congress negotiated its way out of another potential government shutdown earlier this month.

Those negotiations resulted in a deal giving Trump just $1.375 billion for 55 miles of border fences, well short of the $5.7 billion he’d sought for more than 200 miles of steel walls. Along with moving money from other accounts, the emergency declaration would give the president access to some $6 billion more for border barriers. During the 2016 campaign, Trump repeatedly said Mexico would pay for the wall.

Despite their evident discomfort with Trump’s course of action, only a small number of senators declared their outright opposition.

“The emergency course is not one I favor,” Sen. Mitt Romney (R-Utah) told reporters Monday evening.

Sen. Marco Rubio (R-Fla.) objected to the concept on “separation of powers” grounds and opposed how money moved from other accounts to pay for the wall would leave those other projects underfunded. “You don’t solve one problem by creating another,” Rubio said.

Romney and Rubio both declined to say how they would vote on the resolution when it reached the Senate.

“I didn’t want it to come to this because I think it’s probably going to get tied up in court,” Sen. John Cornyn (R-Tex.) said. But he blamed Schumer and House Speaker Nancy Pelosi (D-Calif.).

“I lean against the resolution of disapproval mainly because I think this is Schumer and Pelosi’s way of painting the president into a corner,” Cornyn said. “He said all along he wanted the money to do border security, and they basically were determined not to give him what he thought was necessary, so he’s using existing congressional authorizations to access funds. So it’s not like he made this up out of whole cloth.”

Away from the Capitol, there were also signs of mounting GOP opposition. A bipartisan group of 58 former senior national security leaders released a statement asserting that Trump had no basis for an emergency declaration, and 24 former Republican members of Congress signed an open letter opposing the move as an encroachment on Congress’s role.

California and other states have sued to block the emergency declaration, as have some advocacy groups. California Gov. Gavin Newsom (D), was in the Capitol on Monday for meetings on the emergency declaration and other issues. Newsom told reporters that there is no border emergency and declared the whole situation “theater of the absurd.”

“This is pure politics, base politics, and everybody knows it,” he said.

Paul Kane and Rachael Bade contributed to this report.