The Republicans will probably learn from their mistakes (arguably having a President wasn't a mistake but the kind they got was) and their successes (it seems that they didn't poison their own chalice enough to (yet) lose the other parts of government, but it's hard to credit Trump to this fact without assuming N. E. Other in the position would not - save for the old "look over here!" distraction he caused perhaps).
What have the Democrats learnt, though? Except that, whatever you think about the candidates, they managed to pick ones barely more popular than what the Republicans put up (and not enough so, in at least one of those cases).
It's an interesting avenue to ask what both parties have learned from the experience. However, what the Republicans learned may not be positive. They learned what works and what doesn't work, and the bits that worked, they're going to weaponize those going forward. A narcissist having a bad experience with someone else as a result of their pathology doesn't necessarily turn into not a narcissist as a result of learning from the experience, they use the experience to work out how to be a better narcissist. Generally bad experiences cause people to change the absolute least amount that they can, in a way that avoids the bad experience, before they start trying to change themselves in any deeper way. They may be throwing Trump under the bus and throwing Trump's policies under the bus, but that doesn't mean they're rejecting Trumpian tactics and strategy: that just needs to be refined in service to the movement rather than the loose-cannon version that is Trump himself.
If the Democrats learn anything, it should be from both elections. The major criticism of them, in terms of having an ear to the ground is that they scoffed, both times, that any of the stuff Trump was doing or saying would be popular. We've had two elections now where the polls and media did in fact push a narrative where the Democrats were going to absolutely steamroll home, yet it didn't happen either time.
So, first fix the problem of
not knowing what the fuck is going on. Once the liberal / Democratic institutions actually have something in place so they
know what's going on instead of some whitewashed version, that's when you can have a conversation of what to do about it. Maybe they should have picked Sanders in 2016.
Perhaps the same flaws in polling and data collection that lead to Trump's surprise win would have also meant Sanders was the candidate if those weren't so broken. Think about it this way, if the information systems /networks Democrats were relying on were completely wrong on who was actually best candidate out of Trump and Clinton, why would they be any more accurate about who was the best candidate out of Clinton and Sanders?
So yeah, accurate polling methods are half the battle at least. One point is that Rasmussen was widely derided for their "biased" polling methods suggesting Trump was more popular than other polls indicated. Rasmussen was right however, they've been right two times now and dismissed two times now. That's something the Democrats can learn from. Some of Rasmussen's techniques should be considered for other polls to get more accurate information.
In Chomsky's terms, the media represents the interests of the corporate/political class, and the Overton Window that they operate within is what the media operates in. Trump, for all his faults, got outside that Overton Window, and everyone else scoffed that there was nothing there to see. Trump did in fact prove them wrong and if you're going to beat fascism, people have to accept that. If he's that fucking stupid and he got outside your Overton Window and won once, and came a lot closer to winning the second time than he was given credit for, it's time to rethink how things are going.
It's all well and good to say "we can't bring in views outside the Overton Window because that would be giving 'them' a platform" but ... you're going to keep sliding into this shit if you keep doing the same thing. The current strategy seems to be to de-platform anyone outside the acceptable range of opinion. Is that really going to fix the problem that got you here?