Most liberal viewpoints are about as detailed and profound as your average bumper sticker. “Stop Hunger”, “Free Tibet”, “Save the World” and other grandiose campaigns are far more complicated issues than they would ever care to explore. To a liberal, merely wearing the tee shirt or slapping on the bumper sticker means that they are somehow “helping act. In their eyes, the rest of us are “part of the problem”. As such, remember that the simple act of debating or disputing their views places you in their sights as "part of the problem". This is one of the main reasons for the contempt, hostility and outright vitriol that the liberal will throw in your direction.
Liberals are largely unencumbered by burdensome hangups like history, facts or logic. This makes debate rather difficult because all their views are based almost entirely on how they FEEL, not that which is practical or realistic. This is often carried to the point of absurdity. In many cases, liberals will hold views that actually contradict their own views. Try and point out these obvious conflicts, and they resort to the aforementioned name-calling and slander. It’s a textbook liberal debate tactic.