The court is writing on a blank slate as it considers for the first time the Constitution's recess appointments clause. That clause allows the president to fill vacancies temporarily, but only when the Senate is in recess.
The justices heard more than 90 minutes of arguments Monday in a dispute over Obama's appointments to the National Labor Relations Board in January 2012.
Solicitor General Donald Verrilli Jr. warned the court that it would essentially write that power out of the Constitution if it found that those appointments were illegal.
Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.