This is late in coming - it's been done for quite awhile, just been kinda ....busy.
Russia invades Georgia.
World outcry. Sharp, confused (?) criticism by US.
Charge of hypocrisy against US (over Iraq). Duh.
Hypocrisy does NOT weaken an argument, not directly. It lowers persuasive value but the logic still holds.
Three parts to rhetoric: Argument (or logic), the Speaker (how she is perceived), the Listener (his emotions) (the argument being the logic).
So, the US invasion of Iraq doesn't weaken the logic of the simple argument - nations shouldn't invade other nations - but it kills kills kills the Speaker (the US) because no one (few) will take the US argument very seriously due to its own actions of recent history. (Yes the EU is talking too but .... they're not known for action and anyways most of them depend on Russia for their energy).