On stage tonight, Jon M. Huntsman Jr. must back up and expand on his twitter tough talk. Otherwise, he could be remembered as the guy who tossed rocks and hid his hands — not unlike now-departed candidate Tim Pawlenty.

While it’s unlikely that one strong debate performance can rescue the former ambassador to China from single-digit poll numbers, or stop the growing chatter about an early exit, he can begin to make a credible case that he belongs in the top tier if he can stand next to Mitt Romney and call him out on health care, and tell Rick Perry that his views on global warming and evolution are outside the mainstream.

Does he risk becoming the Democratic National Committee’s best surrogate and alienating Republicans in the process? Yes — he’s done it in the past. On Wednesday, he must go further and try to prove the essential argument of his candidacy, which is that the GOP is more moderate than polls show.

The central weakness of Huntsman’s candidacy has always been that in tone, policy and background, he simply may not match the moment. Tonight, he has to forcefully and convincingly argue that he is the man and that this is his moment. It may be a hard task, given the time constraints and the crowded stage. If he whiffs, viewers and voters will go away with this question: Who is he and why is he here?