Just for Devil's Advocate purposes, I think there is a very big difference between saying "Obama is a liar because..." and "Obama is a lying jackass because...".
I personally don't care one way or the other, but some people might. I see no problem with insulting politicians (we insults plenty of other people, so I don't see why politicians would be protected). The problem is that often insulting politicians and political beliefs leads to fighting between posters. It's not the "Obama is a jackass" line that's the problem. It's the poster who responds by insulting the person who said that.