Sanders caucuses with the Democrats, meaning, for all intents and purposes, he is a Democrat. This is not insignificant.

SANDERS: Brian, believe me, I wish I had the answer to your question. Let me just suggest this. I think there are millions of Americans who are deeply disappointed in the president; who believe that, with regard to Social Security and a number of other issues, he said one thing as a candidate and is doing something very much else as a president; who cannot believe how weak he has been, for whatever reason, in negotiating with Republicans and there’s deep disappointment. So my suggestion is, I think one of the reasons the president has been able to move so far to the right is that there is no primary opposition to him and I think it would do this country a good deal of service if people started thinking about candidates out there to begin contrasting what is a progressive agenda as opposed to what Obama is doing. […] So I would say to Ryan [sic] discouragement is not an option. I think it would be a good idea if President Obama faced some primary opposition.

A reader in the comments suggests it’s time for Senator Sanders to put his money where his mouth is.