John McCain is not an easy caricature, but this is getting closer to how I'd like to draw him. This quick little sketch, which turned into a color rendering, gave me a lot of important information about his face. One thing that I'm noticing is that there always seems to be some anger simmering just below the surface. Even when he's smiling, I still can sense that anger. I'm not sure were it comes from. Maybe his experience in Vietnam. At any rate, it's an important thing to remember when you're drawing him.
I also think that his caricature should be a minimalist one. By that I mean you don't have to draw "everything." It's actually better to really simplify his face; using the fewest brush marks to say as much as possible. I actually took quite a few marks out of this in Photoshop before I colored it.
There's a lot going on below his chin - skin folds, flaps, indentations, etc. The scar running down the right side of his face is also a marker of his caricature. His face is kinda plain at first glance, but then you start to see the experience there. Pay particularly close attention to his eyes. These tell quite a bit about his personality.