However I find myself being disagreed with quite often, mostly for not advocating or cheering violence, "by any means possible" change, or revolutionary tactics. It would seem that I'm not viewed as authentically holding my view unless I advocate extreme, violent, or radical action to accomplish it.
Those seem like two different things to me.
Edit: TO COMMUNISTS, ANARCHISTS, OR ANYONE ELSE CALLING FOR THE OVERTHROW OF SOCIETY
THIS OBVIOUSLY ISN'T MEANT FOR YOU.
You can, if you want. If you generally agree with the DNC, labeling yourself a Democrat is a useful label to quickly get your views across. You wouldn't be a Leftist, since the goals and views of the DNC are a maintaining of the Capitalist status quo, but you would be a Liberal, if you want a non-party label to use instead.
I do think familiarizing yourself with Leftist theory would help you make sense of where Leftists are coming from.