Spitting in someone's face is indeed assault. Even when you pretty much only hit them with spittle. So holds Judge McKeown, in a comprehensive review of the various federal cases on spitting (!).
And, yes, we can indeed make a federal case out of it. At least when you spit in front of the VA hospital.
I'm sure that when defendant's counsel, Amy Rubin, was in law school at the University of Montana she was thinking: "You know what? I think I'm going to take that class on 'The Law of Spitting' next semester. I bet that'll come in handy some day."