Back in 1971, an alternate-history (or would it have been alternate-future at that time?) pseudo-documentary called
Punishment Park was made, depicting a US which had been turned into a far-right police state by then-president Richard Nixon. The McCarran Internal Security Act was invoked; left-wingers of all stripes from anti-war singers to Black Panther expies to feminist college activists were being rounded up and tried by 'special tribunals' comprised of authoritarian politicians, military men and other hard-right-wing locals from the same general area; and those who don't choose to serve out long prison terms get sentenced to a three-day romp through the Californian desert, where their objective is to reach a US flag marking their finish line while being hounded by a team of National Guardsmen + deputies and cops from what I would guess are the LASD & LAPD.
Obviously in hindsight, that didn't happen and Nixon wasn't quite the fascist boogeyman the film made him out to be. (Dude wanted to devolve some powers back to the states via New Federalism, implemented affirmative action and even proposed a sort of UBI in his very unfortunately acronym'd Family Action Plan historically) But, my questions/proposal is: how can the US of the 1960s/70s most realistically and quickly become a right-wing dictatorship of the sort shown in Punishment Park? How long can such a regime last, what would it be up to while in power both domestically & in the foreign arena, and what are the probable aftershocks following its decline/downfall, if it ever falls in the first place?
Figuring out a way to include an uneconomical scheme straight out of a YA novel to deal with opponents of the regime akin to Punishment Park (as opposed to more sensible dictatorship tactics, like just shooting them) is not mandatory, but would be a bonus.
A key issue is that right-wing dictatorships are almost always a reaction to the prior implementation, or imminent threat of implementation, of a
left-wing dictatorship.
The constant left-wing pushing of the "evil right-wingers are always JUST ABOUT to stage a coup!" narrative is simply total fear-mongering bullshit. The historical truth is that the right-wing reactionaries are,
as the name implies.... reactionary. They react. Specifically, they react to the radical actions of the left.
So, most credibly, what you need is a far more active left wing that actually comes pretty close to implementing its own plans for seizing (near-)absolute power, with a right-wing coup then being launched as a last-ditch effort to prevent this. It would be similar to the events in Chile that led to Allende being overthrown. (The typical left-wing narrative usually leaves out that Allende ruled by decree, ignored the Constitution, had set up paramilitary death squads, and was about to forcibly dissolve the parliament which had issued a resolution ordering him to cease his abuses. They also leave out that Pinochet and his confederates were asked to act by the supreme court of Chile, which was aware that Allende had zero respect for the constitution and was, in practice, already governing as a socialist tyrant.)
Anyway... have an equivalent to Allende arise in the USA, using the same means, subverting the Constitution to the same degree, and ignoring the resolutions of Congress in the same way... and then you can easily see Congress and the Supreme Court explicitly requisting the military to take action to defend the Constitution and the lawful rule that derives therefrom.
Presumably, the left-wing radicals (organised into paramilitary groups) would resist violently, and if the far left has usurped the media, it would lead to a messy affair. In the subsequent period of fighting and martial law, the military Emergency Government could realistically execute quite a lot of left-wingers without trial, on the rounds that they are traitors and enemy combatants and that martial law is in effect.
The thing is, following the restoration of order, the US military would certainly return command to the civil government, with emergeny elections taking place as soon as possible. The (also recurring) left-wing idée fixe that the US military is full of insane colonels who just want to seize power is absurd.
Naturally, it is realistic that in the aftermath, Congress would pass laws disenfranchising anyone who's ever been a member of a left-wing organisation, banning all such organisations, and putting
very long prison sentences on various forms of terrorism. And there would presumably be a lot of people getting sentenced to prison for their actions during the recent crisis.
But that would be about it.