Although Mr. Yoo’s memos made a mockery of the applicable law, they at least had the approval of the Office of Legal Counsel. In contrast, Mr. Obama’s decision to disregard that office’s opinion and embrace the White House counsel’s view is undermining a key legal check on arbitrary presidential power.

This is a Beltway detail of major significance. Unlike the head of the Office of Legal Counsel, the White House counsel is not confirmed by the Senate — which means that the president can appoint whomever he likes. Some presidents have picked leading legal statesmen like Lloyd N. Cutler, who served both Jimmy Carter and Bill Clinton. But others have turned to personal friends to fill the office. In such cases, it is especially difficult for the White House counsel to say no to a top presidential priority on the grounds that the law prohibits it.

Mr. Bauer is not the only administration lawyer to conclude that the billion-dollar bombing campaign in Libya does not amount to “hostilities” under the War Powers Act. The State Department’s legal adviser and former Yale Law School dean, Harold H. Koh, has also taken this position. This is surprising, since Mr. Koh’s legal scholarship over the years has been highly critical of presidential overreach on matters of national security, emphasizing the importance of Congress’s constitutional powers over war and peace.

If the precedent Mr. Obama has created is allowed to stand, future presidents who do not like what the Justice Department is telling them could simply cite the example of Mr. Obama’s war in Libya and instruct the White House counsel to organize a supportive “coalition of the willing” made up of the administration’s top lawyers. Even if just one or two agreed, this would be enough to push ahead and claim that the law was on the president’s side.

Allowing the trivialization of the War Powers Act to stand will open the way for even more blatant acts of presidential war-making in the decades ahead. Congress must confront the increasingly politicized methods White House lawyers are using to circumvent established law and stop them from transforming it into an infinitely malleable instrument of presidential power.

If Congress does not act, the Constitution’s command that the president “take Care that the Laws be faithfully executed” will become nothing more than an unfulfilled hope on an old piece of parchment.