U.S. President Donald Trump in the Roosevelt Room of the White House in Washington, D.C., U.S., on Tuesday, March 5, 2019.

President Donald Trump on Wednesday urged Senate Republicans to "STAY UNITED" on an upcoming vote about his national emergency declaration.

The GOP lawmakers "are not voting on constitutionality or precedent, they are voting on desperately needed Border Security & the Wall," Trump asserted in a tweet. By framing it as a vote on border security rather than executive power, Trump hopes to put pressure on Republicans who have to appeal to a party base that embraces his tough immigration rhetoric.

Trump's admonition came after multiple Republican senators announced that they planned to vote in favor of a measure to block the president's declaration, likely giving the Senate enough votes to reject the executive action.

"Our Country is being invaded with Drugs, Human Traffickers, & Criminals of all shapes and sizes. That's what this vote is all about," Trump added. "STAY UNITED!"

Trump TWEET Senate Republicans are not voting on constitutionality or precedent, they are voting on desperately needed Border Security & the Wall. Our Country is being invaded with Drugs, Human Traffickers, & Criminals of all shapes and sizes. That's what this vote is all about. STAY UNITED!

Trump has vowed to implement the national emergency anyway, by vetoing the measure if it reaches the Oval Office desk. Neither the Democratic-held House nor GOP-controlled Senate appear to have enough votes to reach the two-thirds threshold needed to override Trump's opposition.

The House passed the bill with 245 votes, well short of the 290 needed to override a veto. The Senate would need 67 votes to circumvent Trump.

All 47 Senate Democrats, along with four Republicans, are expected to support the resolution to terminate the president's use of executive power.

Senate Majority Leader Mitch McConnell has said the Senate will take up the resolution before it starts its next recess on March 18.

McConnell, who has supported Trump's emergency declaration, has conceded that the Senate will pass the measure opposing the president's action. Still, he does not think Congress can overcome Trump's veto, he said.

"I'd be less than candid with you if I didn't say we've had a lot of spirited discussion ... about this whole issue. ... At the end of the day, the president in all likelihood is going to get a measure that he will veto and it will almost certainly be sustained, the veto will be sustained, in the House," he said Tuesday.

GOP Sens. Susan Collins of Maine, Lisa Murkowski of Alaska, Thom Tillis of North Carolina and Rand Paul of Kentucky have already pledged to vote to block the emergency declaration. Trump's efforts to limit defections come as several other Republicans, such as Sen. Lamar Alexander of Tennessee, consider backing the resolution.

If the Senate passes the measure, Trump would issue the first veto of his presidency.

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