That's all you have to do with a lib, as them to prove it or cite sources. Usually they can't come up with anything on their own, which is why they need their leaders to come up with talking points they reapeat ad nauseum.
How many times have liberal friends told you "Bush is an idiot?" It has happened to me several times, to which I respond by asking, "Well what do you mean?"
Ninety-nine times out of a hundred, there will be no good answer to my question and the subject is dropped. Pretty good entertainment if you ask me. All you need to do is ask them to prove their own statements and watch the bumbling commence.