Keep in mind that IANAL.
But.. from the training I received in my past life as a security guard, any time you physically restrain someone (better video of the event) in public, it can be be construed as an arrest. Failure to inform the person detained why you are detaining them (the charge) and of their rights (to remain silent, right to representation, etc.) means that you have performed a false arrest.
It is a misdemeanor charge, must less than an assault would be. That's what I understand of Canadian law, but I believe the US laws are equivalent in this.
If, as Paul Wells suggested, the congressman had punched the student, well that would be assault. So in this case, I would say the congressman was smarter than Mr. Wells as any charges will likely be minor in nature.
HT: David Mader in his twitter