I am not sure what the rules are, but I think from a strategic standpoint, politicians do this in order to not alienate their constituents and want to give the impression of impartiality! (yeah right! Sarcasm) All joking aside, I think this is the mindset of many who get fed up with their party then claim they are independent. Remember we saw this from Justin Amash earlier this year when he renounced being a Republican. Arlan Spector I believe did it before outright switching parties.