I was just sort of kidding about the Clintonites. However, there is a potential downside to expressing your political opinions from a bike.
For what it's worth, they do make actual billboard bikes specifically for advertising. I have seen pictures of them mainly in Europe.
http://www.pedicab.com/pedicabs_billboard.html
http://www.inssa.com.co/eng_bike_billboard.htm
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"be careful this rando stuff is addictive and dan's the 'pusher'."