Obscenity is illegal and all Santorum is doing is suggesting those engaged in such criminal activity be prosecuted.
Child porn involves the forcible exploitation of a child. The child is the victim. Who is the victim with respect to hardcore pornography? The viewer? They consented. The participants? They consented. If you don’t like it don’t watch it. Don’t get the government involved! It would just be a matter of time before the government deemed political speech “obscene” or “hateful”, and sites like this would disappear from the landscape.