There’s a good argument that the modern practice of recess appointment is unconstitutional. Michael Rappaport explains that the presidential power, as originally understood, allowed presidents to fill positions that became vacant between sessions of Congress. Contemporary practice allows presidents to fill positions that are vacant at any time that Congress is not in session: The vacancy doesn’t have to begin during a recess, and recesses in the middle of a legislative session count.

But neither party wants to give up the ability to exercise the modern, extra-constitutional power when it controls the White House, so I don’t expect the Republicans to raise the constitutional point. Tea partiers, on the other hand, might look into this question.