That is an interesting question. Obviously, it is a mild form of vandalism, as it's instantly washable. But still, I think it is vandalism, and that applies whether I agree with the political sentiment or not. (I do in this case.)
We pay a hefty fee to keep our streets, sidewalks, lawns, gardens, etc. clean and cared for. Allowing people to start scribbling graffiti is the beginning of a very ugly slippery slope.
I once lived on a cul-de-sac where some kids got carried away on high school graduation night and sprayed the name of the neighbor's kid who was graduating, with something like "You da man," on the pavement of the cul-de-sac, except they didn't use chalk, they used permanent white paint and HUGE letters. The neighbors (the kid's parents) were really embarrassed, apologized to everyone in the cul-de-sac, and tried to remove the paint, but they couldn't. It stayed there forever, probably is still there about 20 years later, and ruins the looks of the neighborhood. Of course, this is much worse than writing in chalk, but they both are examples of well-meaning vandalism.
And I have to wonder, what is really the point of slogans, political symbols, etc. being written or sprayed all over the place? Is this really supposed to change anyone's political inclinations?