Complex sentence: The U.S. Constitution requires that the most senior federal officers must be confirmed by the Senate before assuming office, but while the U.S. Senate is in recess the President can act alone by making a recess appointment.
Split into simpler sentences: The U.S. Constitution requires that the most senior federal officers must normally be confirmed by the Senate before assuming office. However while the U.S. Senate is in recess the President can act alone by making a recess appointment.