I’m going to go with a repeated exposure to propaganda designed to convince people that the election “WAS RIGGED” and sow distrust in our system of government. The left certainly has some questionable positions, but the “stop the steal” movement is not rooted in fact.
But it isn't inevitable that people update their priors. Many people don't when exposed to advertising, for example. If we recognize propaganda, then maybe we can do the same.
The problem with that is what throwaway894345 is talking about, that people decide that you're propaganda and won't listen, even if they are the ones who have believed the propaganda lie.
Now, that wasn't a view that was largely held on the right. In fact, I suspect it still isn't. The "post truth" on the right are people who still believe in truth, believe that they have it, and won't listen to any evidence to the contrary. In fact, maybe this is the consequence of the "post truth" left - if the right believes that the left believes what the left says about speech, then the right has no reason to trust any truth claim coming from the left.
But the change may in fact be as you say, that the post-truth people grew on the right. Now you can't sway chunks of either side with facts.
Through this lens, my own theory of what's happened is that a few conservative news outlets -- conservative talk radio, Fox News, etc -- made themselves into an anchor on the political spectrum by strongly holding opinions that would capture a large audience. Christian/pro-life, pro-freedom, pro-free-market, and all the other stances ossociated with conservatives. The left is much less organized because it's trying to capture all voters who don't fit the mold of what is conservative, and there is a ton of diversity outside of conservativism a la LGBT rights, the role of police, the role of unions, how to help minorities, how to help foreigners, the state of US jobs, the importance of the environment, etc. Conserviatives are probably much more unified in their views on these issues than liberals are.
However, as time moves forward the conservative values haven't changed while liberal values have been updating much more quickly, and there's no path for conservatives to come back to the center. If you're pro-life but sympathetic to minorities, you have to choose between the side you see as murdering babies, vs the side you see as not offering help to minorities. You probably side with anti-abortion, then after a while convince yourself that not helping minorities is the right thing to do anyway.
I think a way out is to have more than 2 anchors, e.g. a way to vote pro-gun but also pro-environmental regulation. And I think a way to achieve that is to switch to ranked voting -- it makes 3rd parties more viable so you can vote for the candidate that most aligns with all of your views, which gives people more flexibility for changing their views.
[1]https://phys.org/news/2020-06-theory-political-polarization....
It's the phenomenon of illusory truth. Wiki has some sources [0][1][2]. It seems inherent to discussions on the Internet as it becomes more and more accessible and popular. The only obvious way out seems to be educating people, but not enough entities seem interested in doing so right now. Maybe pessimistically, I don't think people en masse can learn on their own about the dangers of misinformation unless crises occur that directly impact their lives.
[0] https://en.wikipedia.org/wiki/Illusory_truth_effect
[1] https://web.archive.org/web/20160515062305/http://www.psych....
[2] https://web.archive.org/web/20160514233138/https://www.apa.o...
I think that thing is the shift from being anti-science from being uninformed and ignorant to being anti-science being praise worthy since "You can like see through all the BS man". There was once a time when facts and the act of gaining knowledge were really highly praised by Americans but that has shifted. IMO, one of the highest praised virtues today is that of grifting, if you're a scientist and smart then whatever - but if you're an idiot and manage to trick people into thinking you're a smart dude then all praise to you. You beat the system, you're playing 4D chess!
I think (again heavily into my opinion) that this shift occurred because politics shifted to a point where politicians were incentivized to support policies that were actively harming their constituents - discovering that lead was unhealthy and all of the other side effects of pollutants - put a lot of money into the anti-science campaign and let the demonization of intellect flourish.
