So, the white nationalists are a smallish subset of the supremacists and their motivation is personal in the sense that their male hegemony is the real issue. And the function of male hegemony is to counter a pervasive culture of intimidation. Males dominate other males and females to compensate for having been reared in a repressive culture.
America Firsters are different in that their motivation is ideological, not visceral. Their vector of supremacy is derived from the idea that some humans are intellectually superior and ipso facto entitled/destined to rule/govern others. For them, America First is a sort of disguise to hide their own hubris. The American dream whose mundane expression is realized in the ownership of a house and a modicum of material comfort has nothing to do with the ambition to rule.
The ambition to rule has obviously been a component of the EuroAmerican psyche from the start. It rested on the rationalizations of christianity which justified the extermination and subjugation of people who failed to comply with demands. If not the subjugation itself, the rationale for dominion is what the framers of the Constitution tried to invalidate by locating governance in the people and setting religion aside.
Now it has taken almost 250 years for government by the people coming over the horizon and challenging the ad hoc positions of the supremacists who have been acting like rulers illegitimately. It took women becoming materiallly and financially independent with equal suffrage to start a revilution that has been resisted for four decades and was not turned back by claims of temporary supremacy of the executive during war time, nor the designation of the SCOTUS as an ultimate and infallible authority for the simle reason that the court has no access to forces to coerce.
The supremacists are down to their last refuge, the link between the executive and the judiciary, the prosecutors who have discovered absolute power in failing to do their job. William Barr held the fort successfully, but then the people took the helm and replaced the whole crew. Now the question is whether the prosecutor does the job or defers to the principle of supremacy. An idea is perhaps the most seductive force in the world.
Supremacy depends entirely on entities wiling to be ruled. How are they persuaded? By the argument that obedience is the highest good.