I'd go further and say that politics before the 1950's or so is now irrelevant since the parties have effectively swapped ideologies.
But to the larger point, I think you can trace much of this back to the 1970's. In 1978, Newt Gingrich (then just a college professor) gave a speech to a group of college republicans about the problems republicans faced, "One of the great problems we have in the republican party is that we don’t encourage you to be nasty." He went on to tell them what they would have to do to succeed - “raise hell” and treat politics as a “war for power.” His goal was to destroy bipartisan politics in America, so that he could then point to the government and say how evil and dysfunctional they were. Then the electorate would want to dump the democratic majority and bring his party back to power. His idea "was to build toward a national election where people were so disgusted by Washington and the way it was operating that they would throw the ins out and bring the outs in.”
His goal was to use the media to garner power. "If you’re not in The Washington Post every day, you might as well not exist" he told a colleague. His work in Congress was far more destructive than constructive. If the GOP was working on a bipartisan bill with the democrats, he would scuttle it to avoid giving the democrats a win. If the GOP was pushing a partisan bill that wasn't partisan _enough_ he'd attack the republicans sponsoring it as weak. Such efforts, in his view, "represented a culture which had been defeated consistently."
In 1988 he decided to start taking democrats down. So he targeted the democratic speaker of the house, Jim Wright. Gingrich tried to gin up some sort of sex scanal involving an underage page. It failed. Then he tried to attack him for shady lobbing practices. That failed. Finally he found $60,000 that Wright had made selling books, and managed to convince everyone that was a scandal. Wright resigned.
This set the stage for the GOP for the next 30 years. Politics in the US went from boring CSPAN-worthy coverage to front page news involving sex scandals, accusations of treason and constant name-calling - and that has persisted to this day,
Initially democrats had no response for this, since they had no desire to drop what they had been doing that had been working. (Working for both parties, at least up until Nixon.) But then they started losing. And losing a lot. To quote Aaron Sorkin - "You know why people don't like liberals? Because they lose. If liberals are so fuckin' smart, how come they lose so goddam always?"
So democrats started using the same tactics. And it worked as well, although they weren't as good at it. With someone like Trump it works fairly well, because he's so immoral and unprincipled that it was easy. When a former wife accuses him of rape, and 13 other women accuse him of sexual assault, it's not hard to build a case that he is, in fact, something of an evil guy. But on the harder nuts to crack, they are simply not as good at those sort of attacks that the GOP is.
But they are learning. And what Gingrich set in motion over 40 years ago is now coming to fruition - a two party system, where the party that can be the most nasty wins.
(Which is one of the reasons I am now an independent.)