President Obama’s attempt to unilaterally appoint three people to seats on the National Labor Relations Board and Richard Cordray to head the new Consumer Financial Protection Bureau (after the Senate blocked action on his nomination) is more than an unconstitutional attempt to circumvent the Senate’s advise-and-consent role. It is a breathtaking violation of the separation of powers and the duty of comity that the executive owes to Congress.

Yes, some prior recess appointments have been politically unpopular, and a few have even raised legal questions. But never before has a president purported to make a “recess” appointment when the Senate is demonstrably not in recess. That is a constitutional abuse of a high order.

As a former U.S. attorney general and a former Office of Legal Counsel lawyer who provided advice to presidents on recess appointment issues, we have defended and will continue to defend the lawful use of the recess appointment power. Although originally conceived by the Framers for a time when communicating with and summoning senators back to the Capitol might take weeks, it is still valid in a modern age — but only as long as the Senate is in recess. Not only was the Senate not in recess when these purported appointments were made, it constitutionally could not have been.

Article I, Section 5, of the Constitution states that neither house of Congress may adjourn for more than three days without the consent of the other house. The House of Representatives did not consent to a Senate recess of more than three days at the end of last year, and so the Senate, consistent with the requirements of the Constitution, must have some sort of session every few days.

The president and anyone else may object that the Senate is conducting “pro forma” sessions, but that does not render them constitutionally meaningless, as some have argued. In fact, the Senate did pass a bill during a supposedly “pro forma” session on Dec. 23, a matter the White House took notice of since the president signed the bill into law. The president cannot pick and choose when he deems a Senate session to be “real.”

It does not matter one whit that most members of Congress are out of town and allow business to be conducted by their agents under unanimous consent procedures, because ending a session of Congress requires the passage of a formal resolution, which never occurred and could not have occurred without the consent of the House.

President Obama is not the first to abuse the recess appointment power. Theodore Roosevelt did as well, but for almost 90 years the executive branch has generally agreed that a recess as recognized by the Senate of at least nine to 10 days is necessary before the president can fill any vacancies with a recess appointment.

When Senate Majority Leader Harry Reid (D-Nev.) kept the chamber in pro forma sessions at the end of the George W. Bush administration, he declared that was sufficient to prevent Bush’s use of the recess appointment power. Reid was right, whether or not his tactics were justified.

President Obama’s flagrant violation of the Constitution not only will damage relations with Congress for years to come but will ultimately weaken the office of the presidency. There eventually may be litigation over the illegal appointments, but it will be a failure of government if the political branches do not resolve this injustice before a court rules. The White House has refused to admit or deny whether it received advice from the Justice Department (or overruled its advice), which is telling enough, but its campaign-style announcements about the propriety of its actions are not legally credible.

Congressional leaders of both parties must vigorously (though thoughtfully) defend their prerogatives. Senators could filibuster all presidential nominations, as Sen. Robert C. Byrd did in 1985 over a lesser recess appointment issue, until Obama rescinds these wrongful appointments. The House or Senate could condition all “must-pass” legislation for the remainder of 2012 on an agreement to rescind these appointments. The House also could require the attorney general to produce legal justification and testify at oversight hearings.

If Congress does not resist, the injury is not just to its branch but ultimately to the people. James Madison made clear that the separation of powers was not to protect government officials’ power for their sake but as a vital check on behalf of individual liberty. To prevent future tyrannical usurpations of power, Congress must act to redress this serious threat to our liberty.