WASHINGTON — President Donald Trump on Thursday pledged to veto a resolution that would terminate his national emergency declaration just hours before the Senate is set for a showdown vote on the measure.

"A big National Emergency vote today by The United States Senate on Border Security & the Wall (which is already under major construction)," Trump wrote. "I am prepared to veto, if necessary. The Southern Border is a National Security and Humanitarian Nightmare, but it can be easily fixed!"

"I don’t know what the vote will be, it doesn’t matter," Trump told reporters later at the White House. "I’ll probably have to veto and it’s not going to be overturned, and we’re going to have our whole thing.”

He added, "The legal scholars all say it’s totally constitutional. It’s very important, it’s really a border security vote. It’s pure and simple — it’s a vote for border security, it’s a vote for no crime."

Sen. Mike Lee, R-Utah, on Wednesday, became the fifth Republican to declare support for the House-passed Democratic resolution to cancel Trump's national emergency declaration — which the president wants to use to pay for a border wall Congress has refused to fund.

Trump tweeted again later Thursday morning, urging GOP lawmakers to stick with him.

"Prominent legal scholars agree that our actions to address the National Emergency at the Southern Border and to protect the American people are both CONSTITUTIONAL and EXPRESSLY authorized by Congress," Trump said on Twitter. "If, at a later date, Congress wants to update the law, I will support those efforts, but today’s issue is BORDER SECURITY and Crime!!! Don’t vote with Pelosi!"

After four GOP senators announced that they would back the resolution — Rand Paul of Kentucky, Thom Tillis of North Carolina, Lisa Murkowski of Alaska and Susan Collins of Maine — Senate Leader Mitch McConnell, R-Ky., last week conceded that it would pass.

Debate started on the resolution around 10 a.m. ET, with the final vote expected in the afternoon.

Sen. Mitt Romney, R-Utah, a frequent Trump critic, announced in a statement ahead of the vote Thursday that he would vote in favor of the resolution.

"I will vote today for the resolution of disapproval," Romney said. "This is a vote for the constitution and for the balance of powers that is at its core. For the executive branch to override a law passed by Congress would make it the ultimate power rather than a balancing power. This is not a vote against border security. In fact, I agree that a physical barrier is urgently needed to help ease the humanitarian crisis at the southern border, and the administration already has $4.5 billion available within existing authority to fund a barrier — even without an emergency declaration."

Sen. Lamar Alexander, R-Tenn., also announced on the Senate floor that he would join Democrats and support the measure, calling the declaration that Trump issued a "dangerous precedent."

“I support the president on border security. I have urged him to build the 234 miles of border wall he has asked for in the fastest possible way by using $5.7 billion already approved by Congress. But his declaration to take an additional $3.6 billion that Congress has appropriated for military hospitals, barracks and schools is inconsistent with the U.S. Constitution that I swore an oath to support and defend.

Never before has a president asked for funding, Congress has not provided it, and the president then has used the National Emergencies Act of 1976 to spend the money anyway. The problem with this is that after a Revolutionary War against a king, our nation’s founders gave to Congress the power to approve all spending so that the president would not have too much power. This check on the executive is a crucial source of our freedom.

This declaration is a dangerous precedent. Already, Democrat presidential candidates are saying they would declare emergencies to tear down the existing border wall, take away guns, stop oil exports, shut down offshore drilling and other leftwing enterprises—all without the approval of Congress.”

Tennessee Senator Marsha Blackburn, on the other hand, says she will vote against the resolution to block the border wall.

“Since Congress gave emergency powers to the executive branch in 1976 under the National Emergencies Act, presidents from both political parties have declared national emergencies in the United States over situations far less dire than the security and humanitarian crisis that is currently plaguing the southern border. The President and Congress must take swift action to secure our border, protect our citizens, and defend our sovereignty. I support President Trump’s declaration of a national emergency and I reject the resolution of disapproval."

Senate Minority Leader Chuck Schumer, D-N.Y., slammed for declaring an emergency "because he lost with Congress."

"He had to trample on the Constitution to continue his fight," Schumer said.

Should Trump veto the measure, it is unlikely that Congress would be able to overturn it.

Trump declared the national emergency weeks after the longest government shutdown in U.S. history, lasting more than a month, which followed Congress' refusal to acquiesce to his demand for more than $5 billion in funding to build a massive wall along the southern border with Mexico. Trump used the emergency declaration in hopes of redirecting billions of federal dollars to build the wall without congressional approval.

Trump on Thursday labeled the Democratic lawmakers opposed to his emergency declaration "'Border Deniers.'"

"They refuse to see or acknowledge the Death, Crime, Drugs and Human Trafficking at our Southern Border!" he tweeted.