A constitutional crisis means the elementary condition for political order snaps. That condition Aristotle calls peace.
Peace is the transfer of power without bloodshed.
In this sense any political system is constitutional, legitimate if it avoids bloodshed at the point of transfer. In a voter based system this is an election. He means the transfer of power in the US will at this point strain the constitutionality of the American system. There could be bloodshed. The ancients' understanding of things was you had to make the stakes for good government higher than just peace.