It's very simple. Putin wants Sanders to be our nominee. The GOP wants Sanders to be our nominee.

[Progressive dog]

24. I blame Sanders for what he is doing,

He is running in a primary in a political party that he doesn't support.

Their take on Sanders’ role: In 2008, he got on board with Obama late. In 2012, he was either attacking the president from the left or doing little to get him elected. As for “vigorously” supporting him, they said, Sanders was not someone they thought of as a top surrogate.
“Every indication we had was that he was considering a primary challenge,” said one senior official from Obama’s 2012 campaign.
“Being annoying,” another said when asked to recall how they remembered Sanders in 2012.

https://www.buzzfeednews.com/article/evanmcsan/the-obama-campaign-remembers-2012-very-differently-from-bern

Sanders, 73, has been preaching socialism for nearly half a century, and he cites Eugene Debs, the five-time presidential candidate of the Socialist Party of America, as his hero. But he hasn’t always embraced the label.

I myself don’t use the word socialism,” he said in 1976 in the Vermont Cynic, a student publication at the University of Vermont, “because people have been brainwashed into thinking socialism automatically means slave-labor camps, dictatorship and lack of freedom of speech.”

https://www.politico.com/story/2015/07/14-things-bernie-sanders-has-said-about-socialism-120265