You should be able to wear whatever kind of shirt you want as long as it is not considered offensive, and I'm talking the general standard for offensive, not political. Just like when Bush (W) and many Republican candidates presently are barring people with any indication that they might be from the other side from attending public rallys. They even have hired private security firms and bikers who may handcuff you (private security) and escort you from the immediate area. I don't know how that could be legal. There is a phenomima where politicans from both parties are trying to avoid hard questions and any scrutiny of their political positions.