“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 newspaper. “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.”

Paul joins fellow Republican Sens. Susan Collins (Maine), Lisa Murkowski (Alaska) and Thom Tillis (N.C.) in opposing Trump’s move, a reflection of some resistance within the GOP to what lawmakers see as executive overreach and a test of the constitutional separation of powers.

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The disapproval resolution has already passed the Democratic-controlled House and requires a simple majority to pass the GOP-led Senate. Fifty-three senators caucus with Republicans and 47 caucus with Democrats, meaning that four Republican defections would be enough to ensure passage.

While the resolution is likely to clear the Senate, an embarrassing rebuke to Trump, lawmakers in both chambers lack the votes to override a threatened presidential veto.

Senate Majority Leader Mitch McConnell (R-Ky.) spent weeks warning against a national emergency only to declare his support for the move last month. McConnell faces reelection next year and there is concern within the GOP about being forced to choose between Trump and their self-described opposition to executive overreach.

Republicans worry that in supporting Trump, they will be giving approval to a White House power grab that circumvents Congress’s constitutional power over spending. But if they oppose it, they face the wrath of not only the president but his political base — and possibly a primary challenge.

Sen. Lamar Alexander (R-Tenn.), who has been critical of Trump’s emergency declaration, delivered a floor speech on Thursday in which he outlined what he described as an alternative way for the president to get the money he wants to build his wall. But Alexander has declined to say how he would vote on the resolution.

Numerous other GOP senators have also expressed reservations about Trump’s move, among them are Sens. Ted Cruz (Tex.) and Marco Rubio (Fla.), leading to widespread expectations that the resolution will easily pass the Senate.

The Senate is poised to vote on the measure later this month.

Asked about Paul’s decision, his spokesman, Sergio Gor, said it “speaks for itself” and declined to elaborate further.

Trump has said he would veto the legislation, and the vote margin in the House last week — 245 to 182 — was well short of the two-thirds majority that would be required to override his veto.

Nonetheless, the resolution represents a blow to Trump’s Feb. 15 move to declare an emergency after Congress balked at giving him the money he demanded for his border wall. Trump’s declaration allows him to access $3.6 billion in funds allocated for military construction projects. That money would be tapped after the administration exhausts funding from other sources, including $1.375 billion provided by Congress; $2.5 billion from a Pentagon account for countering drug activities that the administration can access without an emergency declaration; and $601 million from a forfeiture fund in the Treasury Department.

During an interview last week with Fox News’s Sean Hannity, Trump urged Republicans not to back the disapproval resolution and said those who do so will “put themselves at great jeopardy.”

“I think that really it’s a very dangerous thing for people to be voting against border security,” Trump said.

Trump also raised the issue on Saturday during a wide-ranging speech to the Conservative Political Action Conference that stretched on for more than two hours. He dismissed Republican criticism that his declaration could set a precedent for future Democratic presidents, arguing that the solution was simply for him to be reelected in 2020.

“They’re gonna do that anyway, folks,” Trump said. “The best way to stop that is for us to win the election.”

Trump repeatedly said during the 2016 campaign that Mexico would pay for construction of the wall. Instead, he has asked American taxpayers to finance it.