When we call Obama or Bush Hitler,we’re making an analogy. For an analogy to work,it must have a shared understanding of the premise of the argument: people have to know why the analogy works.
This becomes clear in the obvious format (where analogies lose all humor value) of "Obama’s Health Care proposal" is like "Hitler" because of X,Y,Z. Adding the whole "because" clause in there really ruins the fun of the analogy because it explains its basis: the fact that it has a generally accepted and obvious shared premise.
Of course,if it doesn’t have a shared premise,then the "because" clause comes in quite handy in explaining what was supposed to be obvious,your premise – thus proving that you have failed in your analogy. There is no shared premise. It has also destroyed any humor content therein.
So,you can skip the analogy portion of the argument,since it wasn’t all that funny anyways,and skip right to the because clause explaining the basis for the argument,which should clarify a few things on all sides.