I like the notion of having two sets of terms. "Liberal" and "conservative" for people who wish to describe either their general approach to politics or to a particular issue. These terms allow people to accommodate having different approaches to certain things. I am happy to be labeled liberal overall, as it describes many of my views--liberal on government's role in guaranteeing children basic necessities, conservative on the death penalty (but liberal in thinking the government has a higher duty than it is showing to guarantee it is fairly administered with proper legal representation and resources, etc), liberal on progressive taxation (progressive is a recognized economic term as used here, not a political approach), conservative about the need to reduce deficit spending, conservative on 2nd amendment but liberal in thinking that reasonable regulation/licensing and safety training are constitutional, liberal on reproductive rights and abortion but willing to concede that the conservative view has merit so the issue cannot be settled by court or legislative fiat, liberal about the need for international cooperation but conservative in believing we need a three-ocean military presence....
"Alt-left" and "alt-right" seem to me to be labels that can be used to describe people who have unyielding uniform adherence to a set of extreme beliefs and who approach political discussions with the view that no one else is worth listening to. These are the people who spread conspiracy theories, attribute fascist or socialist thinking to anyone disagreeing, even if they are generally in agreement on other issues. These are the people who are most incensed about pretty much everything, and they overwhelm discussion with noise and disparagement. They use labels about people to dismiss ideas that are brought up. An example is the inability of most Trump supporters to recognize that there are conservative icons who believe Trump is pursuing policies anathema to conservatism (George Will, John McCain, Kathleen Parker, the recently departed Charles Krautheimer)--the most notable characteristic of an extremist is to refuse to even consider that other people have brains, too. Similarly, the rise of speech codes on college campuses is the result of giving in to shrill demands for protection that refused to listen to almost a century of academic scholarship that treasured academic freedom and eventually established campuses as havens of free speech and scientific inquiry.
Extremists have long been with us but generally their views have failed to carry the day in post-WW2 politics.The problem is that the alts on both ends are now, through the magic of the Internet, both organized and mobilized, and, due to the demise of traditional news, they have been given apparent credibility. The media gives far more attention to an Alex Jones screed than it does to a speech by a General Powell, and it thrusts videos of World Bank protestors into our faces for days at a time without ever reporting on what the World Bank actually does. (Conservatives criticicize the MSM as replacing news with editorials, but I think that is wrong--the MSM has replaced news with sensationalism as it has replaced editors with Web supervisors and marketing analysts.)
As the extremists shout down the people who oppose them, they seem sometimes to save the loudest and most vicious attacks on people closest to their end of the spectrum. If you aren't a Trump Republican, you can't be a Republican deep down inside so you shouldn't be listened to. If you support BLM but think NFL players are waging a counter-productive battle, you aren't a true progressive or worse, you might be a closet racist (God forbid you ever laughed at a Flip Wilson joke).
Where am I going? Right here: the extremists are highly motivated, and have taken advantage of the relative non-activism of moderates in both parties, in order to dominate primaries. With as few as 5% of voters participating in primaries, the extremists have cashed in on a huge opportunity to decide who runs in general elections. This has resulted in a race to the fringes by candidates that is almost comical--here in SC, the Republican candidates for governor all ran ads pointing out every time they refused to compromise with anybody about anything, including the incumbent proudly trumpeting how he vetoed lots of legislation passed by the Republican legislature! After the primaries are over, we get 2 candidates who are then incapable of moving back to the center because of the tight bonds held over them by the activists--the extremists--to whom they owe their primary victories.
When anyone willing to compromise gets labeled a "dirty compromiser" very little can get accomplished. Each party will spend its time trying to undo the other party's accomplishments. Our country was founded on the basis of recognizing that there are many disparate branches of political thought. Compromise was always previously believed to be the best approach of achieving the broadest success. Only if we as a whole decide it is more important to build something, rather than defeat somebody, will we return to the greatness that defined our unique experiment as a country.