Over on the catch all,
@ELA stated that Trump shifted American politics away from running towards the middle.
Others have blamed social media.
I think there are many causes but I think this predates Trump by around 20 years.
So back up to the 2000 election. Of course we all remember Florida and Ohio and hanging chads and six weeks of "too close to call" but lost in the commotion of all of that is, I think, the beginning of the shift away from ELA's notion of running towards the middle.
In the roughly week prior to the election it looked like Bush was going to get a solid win. Gore needed to very nearly run the table in the swing states of that time and he very nearly did. Bush actually underperformed relative to the last polls.
In the post-election review period it came out that Gore had very nearly pulled off the upset NOT because he "ran to the middle" but because his campaign had operated a VERY effective voter registration and GOTV effort among likely Democratic Voters. These newly registered voters didn't show up in the polls because most polls are of "likely voters" and they generally determine who is a likely voter by asking whether or not the person voted in the last election. Those say they did are likely voters, those who say they didn't, aren't. Thus, all of Gore's new voters didn't show up in polls of "likely voters".
Fast forward four years:
Whatever you think of Bush himself, his people weren't idiots. They saw what happened in 2000 and said "two can play at this game". In the runup to the 2004 election I suddenly started seeing Republican Political operatives showing up at Church events, Gun Shows, basically anywhere that there was a crowd of likely Republican Voters.
Now going back a bit:
What ELA referred to is known in PolySci as the Median voter theorem. To explain it, suppose that Democrat
@SFBadger96 and Republican
@medinabuckeye1 are running for B1G Board Political Representative. Now suppose (for simplicity) that only the 11 most frequent posters are going to vote. If we line up those most frequent posters from L->R then number them with #1 being the most left-wing and #11 being the most right-wing then according to the theorem, whoever gets voter #6's vote will win. Basically, if
@SFBadger96 leans far enough right to get voter #6 he'll still get 1-5 because their only other choice is to vote for a Republican and similarly, if I lean far enough to the left to get voter #6 I'll still get 7-11 because their only other choice is to vote for a Democrat. #1 and #11 will "hold their nose", grumble about "DINO's/RINO's" and vote for the guy on their side.
Gore's strategy in 2000 was an end-run around the middle voter theorem. Instead of fighting hard over voter #6 he attempted to engage non-voters #0, #-1, #-2, and #-3 to get them to vote and effectively push the middle from voter #6 to voter #4.
The bottom line is that Gore's strategy was effective (not quite effective enough for him) because it is generally easier to convince non-voters who already agree with you to vote than it is to convince voters who disagree with you to completely change their worldview.
At least since 2004 both Parties' primary focus has been on getting their people to the polls rather than convincing moderates that theirs is the best way forward as it was prior to that.
Social media, IMHO basically poured gasoline on the fire but the fire was already raging.
Social media makes it MUCH worse because people stay in their own bubbles were (from my example below) EVERYONE agrees that she really "got" me.
I've seen literally countless videos on Facebook and YouTube where the hook is basically "watch this liberal get owned" or "watch this conservative get owned" where if you actually watch the video with anything close to an open mind, nobody got owned. The two sides stated their position. You could literally take the EXACT same video on a given issue and package it for Liberals as "Watch this conservative get owned" then package it for conservatives as "Watch this liberal get owned".