Sen. Rand Paul (R-Ky.) said Saturday that he will vote to block President Donald Trump's declaration of a national emergency at the southern border—potentially providing the tipping-point vote that would allow Congress to block Trump's attempt to use the declaration to obtain funding for a border wall.

"I can't vote to give extra-Constitutional powers to the president," Paul said during a speech at a Republican Party dinner in his home state on Saturday night, 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."

Trump declared a national emergency along the southern border on February 15, shortly after the conclusion of a weeks-long government shutdown that failed to convince Congress to grant his request for $57 billion to build about 230 miles of fencing. The emergency declaration seeks to redirect about $3.6 billion in defense funding already appropriated by Congress to the border wall project.

But the House of Representatives voted last week to terminate the national emergency declaration. House Democrats voted unanimously for that resolution, and 13 House Republicans broke with the White House to support it.

It's been less certain whether that resolution can pass the Republican-controlled Senate, but Paul's stated opposition to Trump's emergency declaration likely means it will. Republican Sens. Susan Collins (R-Maine), Lisa Murkowski of (R-Alaska) and Thom Tillis (R-N.C.) have already said they would vote to block Trump's declaration. With all 47 Senate Democrats expected to oppose it as well, four Republican votes would be enough to secure the resolution's passage. With a number of other senators on the fence, it seems likely the final tally will be higher than the 51 votes necessary for passage.

That likely will not be the end of the drama over the border wall funding, as Trump has promised to veto the disapproval resolution. Overriding that veto would require 290 votes in the House—45 more than the resolution received last month—and 20 Republican votes in the Senate to reach the 67-vote threshold.

Even so, congressional opposition to the emergency declaration is a welcome rebuke to the Trump administration's executive overreach in this instance. The emergency declaration is an obvious end-run around Congress and could set a precedent to be exploited by future presidents eager to spend money on projects not approved by Congress.

Leaving aside those broader issues, the emergency declaration is deeply flawed on its own. The president may have the authority to redirect spending due to the emergency declaration, but he does not have the authority to seize private lands or to use military funding for a civilian construction project—even under the vague and broad powers granted to the executive by the National Emergencies Act.

It remains to be seen how Paul's opposition to Trump's emergency declaration will affect the senator's relationship with the White House. In recent months, Paul has made a concerted effort to bend the president's ear on foreign policy and has praised Trump's efforts at ending America's decades-long conflicts in the Middle East—though he also broke with the White House by voting against the confirmation of Attorney General William Barr. Paul cited concerns about Barr's stance on warrantless surveillance.

On Saturday night, Paul reportedly praised Trump in his speech to about 200 Kentucky Republicans, the Bowling Green Daily News reported, before turning abruptly to announce his opposition to the president's emergency declaration.

"We may want more money for border security, but Congress didn't authorize it," Paul said. "If we take away those checks and balances, it's a dangerous thing."