Washington (CNN) Republican Sen. Mitt Romney announced Thursday that he'll vote to overturn President Donald Trump's national emergency declaration, saying his decision was made to defend the Constitution.

"This is a vote for the Constitution and for the balance of powers that is at its core," the Utah Republican said in a statement ahead of the Senate's vote on the resolution. "For the Executive Branch to override a law passed by Congress would make it the ultimate power rather than a balancing power."

Romney joins at least five other Republican senators who have said they will also vote for the resolution of disapproval, which Trump says he plans to veto. The Senate vote is scheduled for Thursday.

The Republican senator, who famously feuded with Trump during the 2016 election, said he's "seriously concerned that overreach by the Executive Branch is an invitation to further expansion and abuse by future presidents."

"We experienced a similar erosion of congressional authority with President Obama's unilateral immigration orders -- which I strenuously opposed," Romney said. "In the case before us now, where Congress has enacted specific policy, to consent to an emergency declaration would be both inconsistent with my beliefs and contrary to my oath to defend the Constitution."

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