"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."
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 — just past the majority needed.
The House has voted to derail the action, and if the Senate follows later this month, the measure would go to Trump for his promised veto.
Many lawmakers opposed to the emergency declaration say it tramples Congress' constitutional power to control spending and would set a precedent for future Democratic presidents to make such a declaration for their own purposes. They also are concerned Trump would siphon money from home-state projects for the barrier construction.
Under a national emergency declaration, Trump would divert $3.6 billion from military construction to erect more border barriers. He also is invoking other powers to transfer an additional $3.1 billion to construction of a wall.
The Associated Press contributed to this report.