This is an old liberal tactic. It asserts their position as being some kind of self-evident truth, and agreement with it a litmus test of rationality. The term for it is dogma, and it has no place in the free debate and exchange of ideas because it's purpose is to prevent it. Unfortunately it seems to be gaining in popularity on both ends of the political spectrum.
It's older than the hills - you'd have heard the same thing in the Roman Senate, and in ancient Greece. It's still amazing how many people can't manage to see through it, though.