Have we seen more violence coming from left-leaning groups recently?
Yes, I think that is fair to say.
Has political violence come under the purview of the mainstream left?
No.
Look, the American leftist movement is a big intellectual organism, which is comprised of many different interests and movements. A short list includes: organized labor, immigration advocates, pro-choice groups, communists, anti-fascists, environmental advocates, poverty advocates, LGBT groups, the DNC, black and hispanic supremacists, and many others in between. I include the savory as well as the less-savory just to illustrate the scope of American leftism, and as an admission that not all leftist groups are sanguine. Of these groups, organized labor is the largest by a huge margin. Granted, union membership has been in decline since the 60's, but there are still almost 15 million union members in the country.
What is the size of Black Lives Matter? What is the size of the Black Bloc? The Berkeley editorial board? The first one is a trick question because it's not an organized movement and is itself a mishmash of ideologies, and even then the violent elements are only part of it. The second one is in the thousands, perhaps. The third is made up of 16 people, excluding administration staff.
What I'm trying to illustrate is that until advocates of this kind of political violence reach a critical mass on the left, it will continue to be in the domain of the fringe.
What
@TLDR20 and others are trying to illustrate is that we don't apply the same standards to American conservatism as we do to the left. Why don't we make a similar list for the right? We've got business interest groups, evangelicals, Constitutionalists, Libertarians, the GOP, States Rights groups, anti-immigration, militias, the Tea Party, pro-life groups, and others. During the Tea Party ascendancy, did the left say "Look, this is now conservatism"? Do we define conservatism through a mixture of militias, anarcho-capitalists, and white nationalists? No, we do not. Nor should we.
Now, if you suddenly have an influential person of power (either politically or economically) on either side begin to advocate violence, then you can start to figure that violence is in the process of being legitimized. Like, if Obama or Clinton came out tomorrow and said "Yeah, let's burn this place down," then you would have a fair argument. The same principle would apply if the Kochs, Peter Thiel, or Newt Gingrich began speaking on behalf the Volksfront.
