GOP Sen. 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.) said on Monday that he will support a resolution to block 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 on the U.S.-Mexico border.

"I would vote in favor of the resolution disapproving of the president’s national-emergency declaration, if and when it comes before the Senate," Tillis wrote in a Washington Post op-ed.

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Tillis's decision comes a day before the House is expected to take up the resolution to block Trump's national emergency declaration. Because Democrats control that chamber, it's expected to pass and kick the fight to the Senate.

Tillis's decision puts Democrats on the brink of being able to block Trump's emergency declaration in the Senate. If the resolution of disapproval passes the Senate it will go to the president's desk, where he has said he will use his first veto.

If all 47 Democrats vote for the resolution they would need to flip four Republican senators in order for it to pass the Senate. The vote needs a simple majority.

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 (R-Maine) 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 (R-Alaska) are both viewed as likely yes votes, which would make Tillis the third Republican defector. Both Tillis and Collins are up for reelection in 2020 and viewed as targets as Democrats look for ways to pick up seats.

Collins told reporters in Maine last week that she would vote for a "clean" resolution to block Trump's emergency declaration.

Murkowski told an Alaska TV station on Friday that she would "probably" vote for it, adding that "if it's what I have seen right now, I will support the resolution to disapprove."

Trump declared the national emergency earlier this month that he said allows him to pull from funds that were not appropriated by Congress for the border wall.

He has threatened to veto the resolution if it makes it to his desk.

Trump's decision came in the face of pushback from top Republicans on Capitol Hill, who have fretted about the precedent a national emergency declaration for this purpose could set for a future Democratic president.

Tillis, in his op-ed, wrote that there was "no intellectual honesty" for Republicans if they previously criticized former President Obama's executive actions but don't oppose Trump's on the border wall.

He also noted that several of his Democratic colleagues are running for president and floated that they could use Trump's precedent to try to ram through proposals Republicans are opposed to.

Conservatives "should be thinking about whether they would accept the prospect of a President Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE declaring a national emergency to implement parts of the radical Green New Deal; a President Elizabeth Warren Elizabeth WarrenHarris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda Judd Gregg: The Kamala threat — the Californiaization of America GOP set to release controversial Biden report MORE declaring a national emergency to shut down banks and take over the nation’s financial institutions; or a President Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE declaring a national emergency to restrict Second Amendment rights," Tillis wrote.

He added that, while he supports Trump on border security, the emergency declaration decision was about the separation of powers between the executive branch and Congress.

"As a U.S. senator, I cannot justify providing the executive with more ways to bypass Congress. As a conservative, I cannot endorse a precedent that I know future left-wing presidents will exploit to advance radical policies that will erode economic and individual freedoms," Tillis wrote.

— Updated 7:34 p.m.