Republican senator Rand Paul is throwing his support behind a resolution that would block US President Donald Trump's declaration of a national emergency to build his long-promised border wall.
Paul's move defies a warning from the President and puts the measure on track to passage.
Paul said in a speech at the Southern Kentucky Lincoln Day Dinner that he "can't vote to give extra-Constitutional powers to the president," the Bowling Green Daily News reported.
"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 paper.
"We may want more money for border security, but Congress didn't authorise it. If we take away those checks and balances, it's a dangerous thing."
Paul joins fellow Republican senators Susan Collins of Maine, Lisa Murkowski of Alaska and Thom Tillis of North Carolina in opposing Trump's move.
The disapproval resolution has already passed the Democratic-controlled House and requires a simple majority to pass the 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, lawmakers in both chambers lack the votes to override a threatened presidential veto.
Senator Lamar Alexander, R, who has been critical of Trump's emergency declaration, delivered a floor speech last Friday 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 disapproval resolution.
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 disapproval resolution represents a blow to Trump's February 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 US$3.6 billion in funds allocated for military construction projects.
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.