Bernie Sanders is an Independent.
Edit: in some states it is 'unaffiliated', 'No Party', or 'unenrolled'
Edit 2: Some of you think voting matters. I don't. Some of you think the republican party can be commandeered. I don't.
Deregistering from the republican party is at a minimum about SENDING A MESSAGE.
In Massachusetts it’s called “unenrolled”.
added. thanks!