Mitt Romney this morning ramped up his criticism of Newt Gingrich, calling him an unstable leader and suggesting that the former House speaker could have engaged in “potentially wrongful activity of some kind” by not registering as a federal lobbyist. “He’s gone from pillar to post almost like a pinball machine -- from item to item in a way which is highly erratic,” Romney said at a press conference here, offering a preview of his posture for tonight’s NBC debate.