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 said Thursday that he is "prepared to veto" a Senate resolution blocking his declaration of a national emergency to build a wall on the Mexican border.

The Democratic measure is expected to pass the GOP-controlled Senate later in the day.

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"A big National Emergency vote today by The United States Senate on Border Security & the Wall (which is already under major construction)," Trump tweeted.

"I am prepared to veto, if necessary," he added. "The Southern Border is a National Security and Humanitarian Nightmare, but it can be easily fixed!"

A big National Emergency vote today by The United States Senate on Border Security & the Wall (which is already under major construction). I am prepared to veto, if necessary. The Southern Border is a National Security and Humanitarian Nightmare, but it can be easily fixed! — Donald J. Trump (@realDonaldTrump) March 14, 2019

The House has already passed the resolution, and Democrats only need four Republicans to join them to approve it in the Senate on Thursday.

A number of GOP senators have said they will vote for it, including Susan Collins (Maine), Mike Lee Michael (Mike) Shumway LeeMcConnell shores up GOP support for coronavirus package McConnell tries to unify GOP Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (Utah), Lisa Murkowski (Alaska), Thom Tillis (N.C.) and Rand Paul (Ky.).

Lee on Wednesday became the fifth Republican to say he would vote for the resolution after talks collapsed between the Utah Republican, Vice President Pence and Tillis. Those talks, if successful, would have enabled Lee and Tillis to vote against the measure.

“We tried to cut a deal. The president didn’t appear interested,” Lee told The Hill. “I’ll be voting ‘yes.’"

Lee's decision to vote for the resolution came after the president reportedly called him to pitch his own legislation, which would require Congress to approve future emergencies.

Several other Republicans — including Sens. Marco Rubio (Fla.), Mitt Romney (Utah), Lamar Alexander (Tenn.), Johnny Isakson (Ga.), Ron Johnson (Wis.), Ben Sasse (Neb.) — could also support the resolution of disapproval on Thursday.

Trump in a subsequent tweet early Thursday said Democrats are "Border Deniers" who "refuse to see or acknowledge the Death, Crime, Drugs and Human Trafficking at our Southern Border!"

The Democrats are “Border Deniers.” They refuse to see or acknowledge the Death, Crime, Drugs and Human Trafficking at our Southern Border! — Donald J. Trump (@realDonaldTrump) March 14, 2019

Trump declared the national emergency last month, saying there was a crisis at the border with Mexico and vowing to bypass Congress spend $8 billion to build barriers along the southern border.

Trump's vow to veto a measure blocking the emergency declaration comes after he warned GOP senators on Wednesday that it would be a "bad vote" to block the declaration.

“I said use your own discretion. But I think it’s a bad vote if they go against — I think anybody going against border security, drug trafficking, human trafficking, that’s a bad vote,” he told reporters.

Updated at 8:06 a.m.