The horseshoe theory has always been a little too simplistic and incomplete for American politics. There's no left-wing version of the"tea party" and Keith Olbermann really isn't our Rush Limbaugh. A few years ago, when Jon Stewart held his rally for moderation, I maintained that to suggest that there's parity in partisan religiosity was absurd, but now, given the rise of social media (or my just becoming more aware of it), I'm not so sure of that anymore.
I picked up John Avlon's Wingnuts yesterday after it had been taunting me on my library's new books shelf for the past couple weeks. It focuses mostly on the right's special brand of crazy (with the exception of the aforementioned Olbermann, who gets a meaty chapter), but I think there's plenty of room for criticism of the left's fringe players. Fortunately for us they don't usually get elected to congress or score talking head gigs on CNN. But Tumblr and Twitter has made hashtag activism credible on a mainstream-ish stage and given a platform to those who want to cancel a popular TV show because they don't understand satire. (Of course not all hashtag campaigns are misguided -- some are even pretty amazing like the recent #blacklivesmatter to show solidarity with anti-racist activists during the recent protests over the deaths of Mike Brown and Eric Garner.)
The extreme left and the extreme right do meet somewhere on the outskirts of sensibility, if only in their authoritarian tactics. Recently a women's studies professor gained notority for stealing an anti-abortion sign from protesters claiming that she was "triggered" by it. I think anti-choicers' views are abohorent too, but I absolutely support their right to express them. This is where I usually deviate from the progressive ideology. Trigger warnings and the idea that one should be "protected" from offensive speech may not be censorship in the legal sense, but it's still a form of control I'll never be comfortable with and still call myself a liberal. When the parameters become so narrow and dissent is routinely discouraged, there's very little room for actual discussion.
One more thing that Avlon mentioned, somewhat tangentially related, but as someone with a foot in both white working-class society and middle-class progressivism, this is important to me. He says, "These folks feel like a minority because they fear they are going to be a minority." There's a little more to it than that. It's true that liberals have pretty much abandoned working-class white people who'll vote according their moral compass even when it contradicts their economic interests. (Jonathan Haidt has written a great book on liberal and conservative morality.) It's also important to note that classism is not only tolerated among liberals, but sometimes actively encouraged because we're not like "those people" -- racist, uneducated rednecks. I actually do understand this, and at times I feel as though I have to lean even farther left to prove that I'm not Joe the Plumber.