You make a very keen observation. Until the President responded to the cowardly and deceitful rewriting of history by the Democrats, the debate was characterized as reasonable, necessary, or thoughtful but never bitter.
Until the President responded, it wasn't even debate. It was DEM statements, and the way the media leaves them, the reader tended to take the DEM statements as statements of fact.
So answering the charges 1) causes a debate, that 2) represents an escalation that 3) is bitter (on the part of the responder).
One can watch the same sort of game play out in chatrooms and discussion forums. ;-)