The pervasive mythology of American exceptionalism is essentially a religious doctrine positing that the US is essentially a perfect nation. Consequently, any issues arising within the system are perceived as external threats rather than inherent flaws. This mentality fuels the polarization between mainstream Republicans and Democrats, much like a religious schism. Both sides have a different interpretation of the doctrine and view each other as heretic traitors intent on destroying the nation.
External threat
Incidentally, the notion of a constant external threat fits perfectly into this model. It’s always external forces conspiring to tear down the beautiful city on the hill. Hence why we see constant accusations of people being puppets of Russia or China.