The Democratic Party went off the rails when Obama won. That's when they decided that they had demographics permanently on their side, and chose to aggressively weaponize PC/Identity Politics in myriad ways. They lost their focus.
That also managed to piss off enough people to usher Trump in. Now, to absolutely no surprise, Trump and the GQP are doing everything they can to piss enough people off to vote the Dems back in. Well, what an amazing coincidence.
Back and forth, back and forth. Hardcore ideology makes people do stupid things. The parties just don't learn.