It's possible but hasn't happened because neither side has been willing to upset that precedent yet.  If the court gets packed by whoever is in power pretty soon it's a court of hundreds and a giant farce.  By keeping it small and exclusive both sides have helped maintain the prestige and public respect for the court. If the public loses respect for the court and the whole thing turns farce the country effectively ends.  We're about to that point by other paths anyway so the democrats are signaling they will abandon the 9 judge precedent once they have power again.  They have nothing left to lose.