Sen. Mitt Romney Willard (Mitt) Mitt RomneySenate 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-Utah) said on Thursday 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 emergency declaration to build the U.S.-Mexico border wall.

"I will vote today for the resolution of disapproval. This is a vote for the Constitution and for the balance of powers that is at its core," Romney said in a statement.

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Romney is the sixth Republican senator to say he will vote for the resolution of disapproval, which is getting a vote in the Senate later Thursday.

In addition to Romney, 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), 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), 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 (R-N.C.), Mike Lee Michael (Mike) Shumway LeeMcConnell shores up GOP support for coronavirus package McConnell tries to unify GOP Davis: The Hall of Shame for GOP senators who remain silent on Donald Trump MORE (R-Utah) and 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.) had already announced they would support the resolution. Shortly after Romney made his announcement, 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.) became the seventh Republican to say they will vote to block Trump's declaration.

Underscoring the quagmire Republicans have tried to navigate on Trump's emergency declaration, Romney stressed that he supported Trump on border security but had concerns that his emergency declaration would set a precedent for a future Democratic president.

"I am seriously concerned that overreach by the executive branch is an invitation for further expansion and abuse by future presidents. ... Where Congress has enacted a specific policy to consent to an emergency declaration would be both inconsistent with my beliefs and contrary to my oath to defend the Constitution," Romney added.

With all 47 Democrats expected to support the resolution of disapproval, it has the votes to pass the Senate and force a showdown with Trump, who has pledged he will veto it.

Trump made a series of eleventh-hour pleas to Republicans to vote against the resolution of disapproval, ranging from being open to amending emergency powers in the future to warning they were siding with Speaker Nancy Pelosi Nancy PelosiPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare House lawmakers reach deal to avert shutdown Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill MORE (D-Calif.).

"Prominent legal scholars agree that our actions to address the National Emergency at the Southern Border and to protect the American people are both CONSTITUTIONAL and EXPRESSLY authorized by Congress," Trump said in a tweet Thursday morning.

He pivoted in a subsequent post, adding: "A vote for today’s resolution by Republican Senators is a vote for Nancy Pelosi, Crime, and the Open Border Democrats!"

— This report was updated at 11:16 a.m.