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He’s not a Democrat

Eh, close enough. Romney seems to be the last Republican left in the Senate. So Bernie would be running against fascists. Everybody left of Romney has to be close enough that everybody there should support him or her.

Besides, President Sanders could not create progressive legislation by himself. That's not a President's job. All he could do is sign or veto bills congress sends him, and congress is just not going to be far from the mainstream.