He revealed his view a day before the Democratic-run House was set to vote to annul Trump's emergency declaration, which he made February 15.
In an opinion column posted late Monday by The Washington Post, Tills says he agrees with Trump that there is a crisis at the border.
"Under no plausible assessment of the evidence is there a national emergency today that entitles the president to tap into funds appropriated for other purposes to build a wall at the southern border", the group said in the letter, which will be entreated into the Congressional record Monday.
Four GOP votes are needed in the Senate to pass the measure and sink Trump's declaration. "We have too many smart people that want border security, so I can't imagine that it could survive a veto", Trump told reporters in the Oval Office last Friday.
Tillis faces a potentially competitive re-election next year.
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. "As a conservative, I can not endorse a precedent that I know future left-wing presidents will exploit to advance radical policies that will erode economic and individual freedoms", he wrote.
House Minority Leader Kevin McCarthy, R, said that he wasn't sure how many Republicans would vote for the resolution, "but there will not be enough to override any veto".
Congress has never before sought to cancel a national emergency declared by the president since passage of the National Emergencies Act in 1976.
But he said it's a matter of separation of powers and warned Republicans that declaring states of emergency to get around Congress could come back to bite them.
"We are aware of no emergency that remotely justifies such a step", read a portion of the statement, released Monday, from both Democratic and Republican officials who've worked in the national security realm.
The letter, signed by former intelligence chiefs, senior government officials and Cabinet secretaries, among others, uses forceful wording to clearly define the boundaries of the president's executive powers in this situation.