A mixture of gullibility and cynicism had been an outstanding characteristic of mob mentality before it became an everyday phenomenon of masses. In an ever-changing, incomprehensible world the masses had reached the point where they would, at the same time, believe everything and nothing, that everything was possible and that nothing was true. The mixture in itself was remarkable enough, because it spelled the end of the illusion that gullibility was a weakness of unsuspecting primitive souls and cynicism the vice of superior and refined minds. Mass Propaganda discovered that its audience was ready at all times to believe the worst, no matter how absurd, and did not particularly object to being deceived because it held every statement to be a lie anyhow. The totalitarian mass leaders based their propaganda on the correct psychological assumption that, under such conditions, one could make people believe the most fantastic statements one day, and trust that if the next day they were given irrefutable proof of their falsehood, they would take refuge in cynicism; instead of deserting the leaders who had lied to them, they would protest that they had known all along that the statement was a lie and would admire the leaders for their superior tactical cleverness.
...A mixture of gullibility and cynicism is prevalent in all ranks of totalitarian movements, and the higher the rank the more cynicism weighs down gullibility. The essential conviction shared by all ranks, from fellow-traveler to leader, is that politics is a game of cheating and that the "first commandment" of the movment: "The Fuehrer is always right," is as necessary for the purposes of world politics, i.e., world-wide cheating, as the rules of military discipline are for the purposes of war.
...Without the organizational division of the movement into elite formations, membership, and sympathizers, the lies of the Leader would not work. The graduation of cynicism expressed in a hierarchy of contempt is at least as necessary in the face of constant refutation as plain gullibility. The point is that the sympathizers in front organizations despise their fellow-citizens' complete lack of initiation, the party members despise the fellow-travelers' gullibility and lack of radicalism, the elite formations despise for similar reasons the party membership, and within the elite formations a similar hierarchy of contempt accompanies every new foundation and development. The result of this system is that the gullibility of sympathizers makes lies credible to the outside world, while at the same time the graduated cynicism of membership and elite formations eliminates the danger that the Leader will ever be forced by the weight of his own propaganda to make good his own statements and feigned respectability. It has been one of the chief handicaps of the outside world in dealing with totalitarian systems that it ignored this system and therefore trusted that, on one hand, the very enormity of totalitarian lies would be their undoing and that, on the other, it would be possible to take the Leader at his word and for hm, regardless of his original intent is, to make it good. The totalitarian system, unfortunately, is foolproof against such normal consequences; its ingeniousness rests precisely on the elimination of that reality which either unmasks the liar or forces him to live up to his pretense. (Hannah Arendt, On the Origins of Totalitarianism, pp. 382-84)
To what degree this phenomenon is an accurate depiction of what is developing on the American Right remains to be seen, but for me it's a beginning in trying to think about why Trump can just say anything and not be held accountable for it. The key I think is the hierarchy of contempt as Arendt describes it here. The people in both the Trump organization and in the GOP-dominated congress are all in on the blatant absurdity of what is being foisted on the American public as "truth". They have learned over the last thirty years that they can say any unfounded nonsense (remember voodoo economics?), and that this basic dynamic of gullibility/cynicism will protect them. The higher up they are, the more cynical they are. And the rank and file when asked whether they believe what Trump says admit more often than not that they don't believe him or don't take him literally, that it's politics, that you have to not listen to what he says but look at what's in his heart. Oy. But clearly there is nothing there that they can see there except what they are induced to project into it.
The media and the Democrats are constrained by conventions and norms that make it difficult to call out these lies for what they are, and when they do, they are easily dismissed as partisans or liars themselves. And so our politics have devolved into this kind of chaos where the average Joe has to decide whose lies he thinks are more plausible. In 2008 the Dems won that contest, but this year Trump did, and he did it with a party that controls congress and pretty soon the courts. And as we're seeing in North Carolina, even when the Right loses, it won't accept its loss according to the old democratic conventions.
We do not, of course, live in a totalitarian society, but our government is the the hands of people who have only contempt for the conventions that make a democratic polity possible. This is a pretty scary scenario, not just because there are disagreements on policy, but because we have handed our government over to people who care nothing for the norms of democracy and are willing to construct a self-serving fake reality that will protect them from ever being held accountable.
This is the way they obtain power, and this is the way they will retain it unless some more compelling force for 'reality' confound it in some way. It's hard to see how that could happen, but we're in uncharted waters here, so anything is possible. But the historical precedents are not encouraging. There needs to be a credible voice of opposition, someone who cannot be easily dismissed as a partisan hack. It could be that things have to change significantly for the worse before such a voice could be heard.
The only other reason I have to be optimistic is that perhaps these thugs are not particularly smart, that they are playing a game that they don't quite understand and that they will blunder badly. And maybe there is hope that at least half of Americans are not likely to to be fooled and might resist. I say 'might' because even those who are not Trump voters are so divided, so mired in their confused, subjectivist ontology/epistemology (everybody has a right to his opinion, and everybody's opinion is equal because there is no standard by which they can be judged better or worse), so isolated by their technology, and made so complacent by their material comfort, that it's questionable that they would ever be able to muster any kind of sustained push back. But we'll see.