Checks & Balances is made up of Republican and conservative lawyers, including some who served in recent administrations. George T. Conway III, one of Mr. Trump’s most vocal critics and the husband of the White House counselor, Kellyanne Conway, is one of the group’s most prominent members.

Mr. Barr’s view on executive power is a misreading of the unitary executive theory, said Charles Fried, a Checks & Balances member and Harvard Law professor who endorsed the theory while he was solicitor general during the Reagan administration. In Mr. Fried’s reading of the theory, “the executive branch cannot be broken up into fragments.”

While that branch acts as a unified expression of a president’s priorities, with the president firmly at the helm, “it is also clear that the executive branch is subject to law,” Mr. Fried said. “Barr takes that notion and eliminates the ‘under law’ part.”