Sen. Rand Paul (R-Ky.) said he won’t support President Donald Trump’s declaration of a national emergency to fund construction of a wall along the U.S.-Mexico border, increasing the likelihood that Congress will approve a resolution that seeks to block Trump’s move.

“I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress,” Paul said in a speech at Western Kentucky University on Saturday, 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.”

The libertarian-leaning lawmaker joins GOP Sens. Susan Collins of Maine, Lisa Murkowski of Alaska and Thom Tillis of North Carolina in publicly announcing their opposition to Trump’s emergency declaration. Other Republican senators are also known to have reservations about it, worried that a future Democratic president could point to Trump’s action to justify emergency declarations on matters opposed by the GOP.

Trump announced his declaration last month after failing to convince Congress to give him more than $5 billion to build his promised border wall. The declaration would allow Trump to bypass Congress and use funds specifically appropriated for military projects to begin wall construction.

The Democratic-controlled House voted last week, largely along party lines, to block Trump’s declaration. The Republican-controlled Senate is to vote on the resolution next week.

Assuming all Senate Democrats support the measure, backing from only four Republicans is needed for it to clear the chamber. That would force a presidential veto, the first of Trump’s presidency. Though it is highly unlikely Congress would be able to override the veto ― with support from two-thirds of the members of each chamber needed to do so ― the resolution’s passage would be a political embarrassment for Trump.

Up to a dozen Republican senators have expressed reservations in private about the emergency declaration. Last week, Sen. Lamar Alexander (R-Tenn.) strongly suggested he would not support the president on the matter, and urged him to reconsider.

“I support what the president wants to do on border security, but I do not support the way he has been advised to do it,” Alexander said in a speech on the Senate floor. “It is unnecessary and unwise to turn a border crisis into a constitutional crisis.”

The fate of Trump’s emergency declaration likely lies in the courts. Sixteen states are currently suing the administration over the declaration, and Trump is likely to face additional challenges from the American Civil Liberties Union and a coalition of border property owners.