c) The national calamity.  An event like the Great Depression is so traumatic The People support an unknown "party renegade" like Franklin D. Roosevelt and experimenting with everything becomes acceptable, even throwing the "kitchen sink" at the calamity becomes acceptable.

d) The hardest and most permanent (institutionalized).  Establishing a viable third party to provide that all so necessary third check on the other two parties' power.  As our Founders so well knew, there has to be a third check on power.  Being steeped in classical history and philosophy, they were quite familiar with Plato's "Trinity Doctrine":  https://www.ucg.org/bible-study-tools/booklets/is-god-a-trinity/greek-philosophys-influence-on-the-trinity-doctrine .  Carl von Clausewitz would later use it develop his theory of war:  https://thegwpost.com/2011/11/09/clausewitz%E2%80%99s-remarkable-trinity-today/

Duty calls for it and we owe it to posterity to begin to establish and institutionalize a third party.