Believe it or not, I get asked a lot about how to protect a design on a T shirt. Here is my take on how to do that. 
The first thing that comes to mind is copyright. You can get copyright protection for a photograph (assuming you are not ripping off someone else's photo), or a design, but you can't get a copyright on a short phrase, motto, saying, or the like. To file for a copyright, download a form from the U.S. Copyright Office, fill it out, and send it in with the filing fee.
For a design, you could also get a design patent, which I'll prepare for you for $750. The price eliminates 95% of the askers.
You could file a Trademark on something, if it is used as a trademark, such as Nike, Pepsi, Coke, etc.
As far as I can tell, words like "Vote for Pedro" are not protectable in any way, because they are not a trademark, not protectable by copyright, and not a design. FYI, "Vote for Pedro" shirts are a hot item right now, and are a spin off from the cult hit "Napoleon Dynamite", shot in Preston Idaho. Ask any techno geek or junior high kid.
A big problem with whatever you do to protect designs on a T shirt, is that the barrier to entry into making this product is very low, so 10,000 people could be set up overnight and cranking them out. You can't enforce a patent or copyright against 10,000 people, even if it was valid. So whatever you do, it is likely to be unenforceable.
Comments