President Trump on Wednesday urged fellow Republicans to “stay united” in the face of a Democrat-led effort to nix his declaration of a national emergency at the border.

“Senate Republicans are not voting on constitutionality or precedent, they are voting on desperately needed Border Security & the Wall,” he tweeted. "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!"

NIELSEN DECLARES MIGRATION CRISIS 'SPIRALING OUT OF CONTROL,' WARNS IT WILL GET 'EVEN WORSE'

The House voted last week to block Trump’s emergency declaration, which could help free up billions of dollars to fund a wall on the southern border. The vote was 245-182, with 13 Republicans joining Democrats in voting for the legislation.

While most Republicans in the Senate have supported Trump’s executive order, enough have expressed their disapproval that the measure could pass once it reaches the Senate. Sen. Rand Paul, R-Ky., told a crowd in Kentucky on Saturday night that he "can't vote to give extra-constitutional powers to the president."

"I can't vote to give the president the power to spend money that hasn't been appropriated by Congress," Paul said, according to the Bowling Green Daily News. "We may want more money for border security, but Congress didn't authorize it. If we take away those checks and balances, it's a dangerous thing."

HOUSE REJECTS TRUMP EMERGENCY DECLARATION, SETTING UP POTENTIAL VETO SHOWDOWN

Three other Republican senators have announced they'll vote "no" to a national emergency declaration at the U.S.-Mexico border, too, including, Sen. Susan Collins, R-Maine, Sen. Lisa Murkowski, R-Alaska and Sen. Thom Tillis, R-N.C.

Paul's vote would make it four, and assuming that all 47 Democrats and their independent allies go against Trump, that would give opponents 51 votes — a majority.

But even if the legislation passes, it would almost certainly be vetoed by the White House. In that case, it would require two-thirds majorities in the House and Senate to override the veto -- which would be unlikely.

Trump’s declaration came after Congress OK’d a spending bill that provided $1.4 billion for border barriers at various parts of the border, but much less than the $5.7 billion Trump had demanded. The declaration gives Trump access to an additional $3.6 billion in funding that he can divert to the wall.

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His case was given a boost Tuesday by numbers from the Department of Homeland Security showing illegal immigrant numbers spiking, and the Border Patrol apprehending illegal immigrants at the highest rate since 2007.

“We want to strengthen legal immigration and welcome more individuals through a merit-based system that enhances our economic vitality and the vibrancy of our diverse nation. We also will continue to uphold our humanitarian ideals,” DHS Secretary Kirstjen Nielsen said at a House Homeland Security hearing on Wednesday. “But illegal immigration is simply spiraling out of control and threatening public safety and national security.”

Fox News' Gregg Re, Nicole Darrah and The Associated Press contributed to this report.