The article and several of the comments following share an assumption, that there are two groups of people, those on the left, and those on the right. And that the left is somehow dominated by ignorant or hateful speech. One need not look any further than the comments here to see evidence to the contrary. Neither left nor right has a corner on the market here.

Reasoned political opinion can be discussed amongst those who are able to do so; there will always be those driven by hatred, prejudice, and anger.
The very title and the framing of the article show that the writer is an ideologue more than a journalist, and therefore one should not put too much faith in what he says. One could have easily used the evidence, along with some balancing evidence to show how polarized people's views have gotten and the unfortunate results of that hyperpolarization.

So instead of jumping on the pile and blaming our troubles on damn liberals, add this into other instances when passions were so strong that they threatened to erupt into violence. Bill O'Reilly comes to mind as an example of someone on the right who incites hatred and bigotry to the extent that some are moved to hate speech and perhaps worse.
Looking at some of the points made in earlier comments, one could easily counter with evidence that the other side is and has done the very same things.

But what does that do? Each side simply looks for or manufactures evidence to justify the opinions it already holds. And most of it is driven by hatred of a person or persons; the process so quickly leaves behind any of the ideals and values that inspire us as citizens.

The finger-pointing and insult-hurling is an indulgence we need to leave behind as a society in order to move forward. So it's not a matter of liberalism or conservatism. Both have wisdom for us to use. Let's not look for evidence to vilify each other. Vilifying is a only few short steps away from dehumanizing and committing acts of
violence.
Time to grow up, people!