The problem is that when liberals have a protest rally they always have them in the middle of a liberal enclave like Washington D.C., Los Angelos or NYC. The largest part of the protestors do not have to come in from out of town. I think the only way you could possibly have a significant turn-out is if you had it in a very conservative large city. I'm not even sure there is such a place, but I'm positive that Washington D.C. is out of the question for a significantly large conservative protest.
Big military town in the center of the country.
Not so centralized would be Huntsville, AL.