CNN  — 

Sen. Thom Tillis will be among those Republican senators to vote in favor of a resolution against President Donald Trump’s declaration of a national emergency at the border, the North Carolina Republican made public Monday, increasing the chances that the resolution will be sent to the White House.

Tillis, who is up for re-election in 2020, wrote in an op-ed for The Washington Post that while he favors border security, he is concerned the President has overreached with the national emergency declaration.

“As a U.S. senator, I cannot justify providing the executive with more ways to bypass Congress,” he wrote. “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.”

The measure would block the President from accessing some funds to construct a wall on the southern border, and Trump has promised to veto it should it reach him, which would be his first presidential veto since assuming the office.

The House is set to vote on the resolution Tuesday, and it is likely to pass in the Democratic-controlled chamber.

The measure would then go to the Republican-led Senate and a floor vote would be held within 18 days, according to federal law.

Republicans control 53 seats in the chamber. If all Democrats vote for it, they will still need four Republican votes to help them stop Trump’s national emergency declaration. They have two, now that Tillis and Maine Sen. Susan Collins, who’s also up for re-election in 2020, have said they’ll join.

Other Republicans have also expressed concern about Trump’s decision to declare a national emergency as well.

Sen. Lisa Murkowski, a moderate Republican from Alaska, told a home-state TV station that she would “probably” support the proposal of disapproval.

“Not because I disagree with the President when it comes to border security, (or) certainly national security, but because I think it’s so important that there be clear lines when it comes to the separation of powers,” Murkowski told KTUU in Anchorage. “There’s going to be a great deal of debate as to whether or not the legal authority is there. I would suggest there probably is. The question is … is this over and above the authority that has been granted specifically to the Congress itself?”

To override Trump’s would-be veto, two-thirds of both chambers of Congress would need to vote in favor of the resolution after Trump vetoes it, a steep hill to climb given how popular Trump remains in many states and congressional districts and especially among the base of the Republican party.

Tillis argued that voting on the resolution should not be a question of support for the President and border security. Instead, he said, it is an issue of separation of powers, and he warned Republicans not to “look the other way” because the same tactic could be used by a Democratic president in the future.

He equated his opposition to the national emergency to his opposition to then-President Barack Obama’s executive action creating the Deferred Action for Childhood Arrivals program.

“There is no intellectual honesty in now turning around and arguing that there’s an imaginary asterisk attached to executive overreach — that it’s acceptable for my party but not thy party,” Tillis wrote.

The national emergency is facing legal challenges as well after 16 states filed a lawsuit to block it last week.

Opposition to the national emergency was also expressed Monday in a letter signed by a bipartisan group of former national security officials.

This story has been updated with additional developments Tuesday.