Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) said that he will oppose President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's national emergency declaration to fund a wall along the southern border, as the Senate prepares to vote on a resolution to block it.

Paul, speaking at the Southern Kentucky Lincoln Day Dinner late Saturday, said that he couldn't "vote to give extra-constitutional powers to the president."

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“I can’t vote to give the president the power to spend money that hasn’t been appropriated by Congress. 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," he said, according to the Bowling Green Daily News.

Paul's stance makes him the crucial fourth Republican senator to support the resolution, marking a setback for the administration that would force Trump to use the first veto of his presidency.

He elaborated in a Fox News op-ed, published on Sunday evening, that he would vote for the resolution of disapproval when it gets a vote in the Senate.

"Every single Republican I know decried President Obama’s use of executive power to legislate. We were right then. But the only way to be an honest officeholder is to stand up for the same principles no matter who is in power," he added.

Paul added that Trump's decision to declare a national emergency to get funding beyond the $1.375 billion approved in a recent government funding bill "is clearly in opposition to the will of Congress." He added that if Trump was unsatisfied with the amount provided by lawmakers "his only constitutional recourse" would have been to veto the bill.

"I think the president’s own picks to the Supreme Court may rebuke him on this. Regardless, I must vote how my principles dictate. ... I think he’s wrong, not on policy, but in seeking to expand the powers of the presidency beyond their constitutional limits," Paul wrote.

The Senate is expected to vote on a resolution blocking Trump's emergency declaration before leaving town for a weeklong recess set to begin on March 15.

Supporters of the resolution left Washington last Thursday on the brink of having enough votes to block the declaration and force a Trump veto. With all 47 senators who caucus with Democrats expected to support the resolution, they need to flip four Republicans to clinch the 51 votes needed in the Senate.

GOP Sens. Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (Maine), Thom Tillis Thomas (Thom) Roland TillisThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting OVERNIGHT ENERGY: EPA head questions connection of climate change to natural disasters | Pebble Mine executives eye future expansion in recorded conversations | EPA questions science linking widely used pesticide to brain damage in children Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight MORE (N.C.) and Lisa Murkowski Lisa Ann MurkowskiSenate GOP aims to confirm Trump court pick by Oct. 29: report Senate GOP sees early Supreme Court vote as political booster shot Pence defends Trump's 'obligation' to nominate new Supreme Court justice MORE (Alaska) have already said they will vote for a resolution to block Trump's emergency declaration, which passed the House last week in a 245-182 vote, with 13 Republicans joining Democrats to support it.

Roughly a dozen other Republican senators remain on the fence about Trump's emergency declaration, meaning supporters could pick up several additional GOP "yes" votes before the resolution comes to the floor.

Sen. Lamar Alexander Andrew (Lamar) Lamar AlexanderGraham: GOP has votes to confirm Trump's Supreme Court nominee before the election The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy This week: Supreme Court fight over Ginsburg's seat upends Congress's agenda MORE (R-Tenn.), who is retiring at the end of the current Congress, urged Trump to back down from his emergency declaration, which he warned was creating a "constitutional crisis." But Alexander declined to say how he would vote on a resolution blocking Trump's action.

“I will announce how I’m going to vote when I know what we’re going to be voting on,” Alexander told reporters.

“I learned a long time ago in the United States Senate it’s not wise to announce how you’re going to vote on a vote you may never have to take,” he added.

Trump has said he will "100 percent" veto the resolution of disapproval if it reaches his desk. He also appeared to warn Republican senators who are thinking about opposing him, arguing that supporting the resolution would be a "very dangerous thing."

“I really think that Republicans that vote against border security and the wall, I think you know, I’ve been OK at predicting things, I think they put themselves at great jeopardy,” he told Fox News’s Sean Hannity last week.

GOP aides predicted that the resolution would likely get a vote next week, with Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Senate GOP aims to confirm Trump court pick by Oct. 29: report Trump argues full Supreme Court needed to settle potential election disputes MORE (R-Ky.) setting up four nominations on the floor this week.

Trump announced that he would declare a national emergency the day after Congress passed a government funding bill that included $1.375 billion for physical barriers along the U.S.-Mexico border, well below the $5.7 billion that he requested. He’s hoping to pull together roughly $8 billion for the border wall by combining the emergency declaration, executive actions and the money from Congress.

— This report was updated at 1:06 p.m.