Republican Sen. Rand Paul said he will vote for a resolution denying President Donald Trump emergency powers to build his border wall, a move that could set up the first presidential veto of this administration.

'I can't vote to give extra-Constitutional powers to the president,' Paul told a crowd of Republicans at the Southern Kentucky Lincoln Day Dinner Saturday night, according to the Bowling Green Daily News.

'I can't vote to give the president the power to spend money that hasn't been appropriated by Congress,' he said. '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.'

President Trump has threatened to veto a resolution that would take away his use of emergency powers to build his border wall; it would be the first veto of his presidency

GOP Sen. Rand Paul (left) said Saturday night he could not support the president's move, joining Sen. Thom Tilis (right), who has said the same

GOP Sens. Lisa Murkowski (left) and Susan Collins (right) said they would vote against Trump's national emergency declaration

Paul becomes the fourth Republican senator to cast disapproval on the president's move following Susan Collins of Maine, Lisa Murkowski of Alaska and Thom Tillis of North Carolina.

If all four vote with the 47 Democrats in the Senate it will ensure the legislation denying Trump the use of emergency powers will pass the Senate.

It already passed the House on Tuesday.

Trump has vowed to veto it should it arrive on his desk, which would be the first veto of his presidency.

It takes a two-thirds majority in each chamber of Congress to over ride his veto, which is unlikely to occur in either the House or the Senate.

Several Republican senators took issue with how the president made his move, arguing Trump violated the line that separated the powers of the executive and legislative branches when he declared a national emergency.

Democrats used this argument for their cause in arguing why the GOP should cross the aisle to support their measure ending Trump's move.

'This isn't about the border. This is about the constitution of the United States,' Speaker Nancy Pelosi said last week before the House vote.

It passed the House 245-182 last Tuesday with 13 Republicans joining Democrats in their support.

That number is short of the 290 votes that would be needed to override a veto, but it did move the measure to the Senate, which has not scheduled a vote on the matter yet.

More Republicans could join the rebellion against the president.

GOP Sen. Lamar Alexander issued a warning shot to Trump on Thursday when he took to the Senate floor to express his concerns about the president's use of emergency powers.

'I do not support the way he has been advised to do it. It is unnecessary and unwise to turn a border crisis into a constitutional crisis about separation of powers,' he said.

He stopped short of saying he would support the legislation denying Trump the use of emergency powers.

Several Republican senators took issue with how President Trump made his move, arguing Trump violated the line that separated the powers of the executive and legislative branches when he declared a national emergency

House Democrats, led by Speaker Nancy Pelosi (above), voted to block the president's move to build his border wall on Tuesday night

The White House has worked hard to limit Republican support for the measure.

Vice President Mike Pence attended the Senate Republican luncheon on Tuesday, where the resolution was a key topic.

The GOP is in a tough spot on the matter because many Republican lawmakers had privately warned the president against using emergency powers although several don't want to rebuke him publicly.

'I am very worried, prudential, about the slippery slope that could occur,' Sen. Ted Cruz told reporters last Tuesday afternoon, pausing to carefully consider his next words, 'emboldening future presidents to implement radical policies, contrary to law and contrary to the Constitution.'

Cruz told DailyMail.com that he had spoken to the president about the national emergency resolution and 'time will tell' whether the White House will be able to swing enough Republican lawmakers to sustain a veto.

He said that the GOP had a 'robust and productive conversation' at their luncheon with Pence.

The congressional battle is the latest chapter in a long-running war between Trump and Democrats over border security, immigration policy and the border wall that Trump has pledged to build since becoming a presidential candidate.

He originally promised that Mexico would pay for it, but after Mexico refused, he asked U.S. taxpayers to foot the bill for the project, which Democrats call unneeded and ineffective.

In his first two years in office, Trump's Republicans controlled both chambers of Congress, which under the U.S. Constitution holds the national purse strings, but lawmakers failed to provide the funding Trump wanted for his border barrier.

When Congress, with the House now controlled by Democrats, refused in recent weeks to provide the money he wants, Trump declared an emergency and vowed to divert funds toward the wall from accounts already committed by Congress for other purposes.

That set up a test of the constitutional separation of powers between Congress and the presidency that will likely lead to a court challenge after lawmakers deal with the resolution.

A coalition of 16 U.S. states led by California has already sued Trump and top members of his administration in an attempt to block his emergency declaration.

Trump declared the emergency after Congress declined his request for $5.7 billion to help build the wall.

Congress this month appropriated $1.37 billion for building border barriers following a battle with Trump, which included a 35-day partial government shutdown - the longest in U.S. history - when agency funding lapsed on Dec. 22.