To which I add: 

Both parties are a mess because they wear the blinders of extreme tribalism.  I watch as both sides do the very things they accuse the other side of doing.

The GOP wants to control what is taught in our schools, but so do the Dems.  They each have their list of topics and ideas that are not to be  discussed.  Whatever their methods the end result is the same:  "schools can't say X to students."  As a teacher and a recent student (grad school), I have witnessed intolerance from both sides of politics and both sides of the teacher desk.  Speech itself is a battleground and free speech is in very serious danger--nobody wants to let the other side speak anymore, regardless of what facts or logic they have on their side.   I won't specify the exact topics, but over the past several years I have seen both sides insist we must "follow the science!" for A, and then reject science for B.  Feelings are 'fact' enough, but only for approved topics.

The extremes are now in control of U.S. politics.  That's a sea change from my youth when politicians referred to the "tyranny of the middle (or center)."  The center is gone, absent or mute.  

We can blame the coarsening of our culture, the 'bubble' media, and crooked manipulators.  But blame also goes to the voters themselves.  They put themselves in this position when they register as a member of a political party.  They slap an R or D on their forehead and then cheer for it like a sports team.  Unyielding, uncritical devotion to a sport is harmless.  If you love the Cincinnati Reds right now, the world isn't affected.  But when you apply that to political parties, you can destroy an entire republic.  That mentality strips the voter of all independent thought and action.  They become inflexible and unable to shift when times or the parties change (or see when they're being taken advantage of).  So they warp their minds to justify their party.  They won't admit their party is wrong because it might benefit the other party.  And so accountability is dead.  Sure, they'll hold the other side's feet to the fire but not their own--which is nothing short of corruption.  The only way I see out of this mess is for voters, every last one, to go unregister themselves.  I'm not saying which way anyone should vote, just that they should shed these self-imposed labels and remain flexible. Without those labels, the parties can't just assume your support.  They'll have to work for it.  And that would bring them back towards the center again.  (Open primaries in all states would also help serve that end.)  And there are other contributing factors as well, but I think this is the most important one--and one that we can do something about without needing representatives to do it for us.

Is there much chance this will happen?  I doubt it.  But it's the only path I can see to pulling America back away from the abyss.

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You wrote a lot to make a tired old argumentum ad temperantiam. I /wish/ the extreme left had a foothold in US politics. Instead we have an extreme right party and a center-right party, with conservatives pointing to attempts at inclusion for people other than straight white males, leftists bitching on Twitter, and college students being the smug idealists they always have been as proof that the Democrats are going too far. Whatever dudes. Proclaim the left is going too far to distract from your upcoming sexual assault allegations. 

How is this a riposte to a suggestion that free speech is in danger? While I think you are right in your analysis that the far right is actually a part of a US politics while the far left  holds no place in official government (and the left virtually no place), I think it is a valid observation that both side of the spectrum seem to have little truck these days in engaging with the opposite side. Few seem interested in a marketplace of ideas.