Here’s an alternate theory:

Back then both parties tended to use the “slimy posh salesman” kind of deceit (half lies, misleading statements, promises left unfulfilled with twisted excuses given for it) and what happened was that Republicans, almost against their will (Trump was an outsider) stumbled upon the “strongman saying what people want to hear using straight talking language” technique of deceit.

It was a massive success because people had been fed the same kind of posh-saleman deceit (with pretty much all the media having gone along with it and turning into de facto propaganda outlets) for decades, found the new sort of discourse refreshing and comparatively trustworthy (even though it was still bullshit, just delivered differently, because it did not sound at all like the usual bullshit it seemed trustworthier to many and all the media that had chosen sides and gone along unquestionigly with the old style of deceit wasn’t trusted anymore so all their denouncing of the “other side’s lies” was ignored)

Whilst the Republicans did adjust to that unexpected success quickly, the Democrats did not and just kept using with their usual deceit techniques spread via the media-outlets to which lots of the electorate had built a resistance to, and acting to favour the usual people when in government.

So here we are today with a lying populist getting power for a second time in the American Two-Party system.