Despite the very vocal urgings of many supporters, it seems extremely unlikely that Sanders will run in the General Election, if he doesn't win the nomination of the Democratic Party. Sanders has committed himself on numerous occasions to endorsing whomever the Democrats' nominate, and to work hard to insure Trump isn't elected. He has also repeatedly said that whatever happens, he will not be a "spoiler."
Sanders has declined to respond directly to Stein's invitation to discuss collaboration, but The Los Angeles Times reported that Sanders was asked if he would consider Stein's offer before he took the stage at a rally in Santa Monica. His answer was "no." However, Stein's overture may well be intended more for his supporters than to Sanders himself.

Logo of the Green Party of the United States
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If Sanders doesn't get nominated, the Green Party could find itself in the kind of historical moment it has been waiting for since its founding as a national party twenty years ago. (It's roots go back another ten years.) https://en.wikipedia.org/wiki/History_of_the_Green_Party_of_the_United_States
Soon, there could finally be a relatively large base that is fed up with the Democrats and ready to take a chance on the Green Party. The party could not only receive an unprecedented amount of votes in the General Election, perhaps surpassing Ralph Nader's vote total of 2000, but an injection of new activists, including many millennials.
With so much at stake, one might think that Green Party activists would be pushing for Clinton to win the Democrats' nomination. To the contrary, many Greens are supporting Sanders. The Greens have been hit hard by voters changing their registration, at least temporarily, to vote for Sanders. Even some who've remained with the Greens have supported Sanders' campaign. However, whether they're newly ex-Greens, or Green stalwarts, they are of one mind about Clinton -- they don't trust her, see her as part of the establishment, and view her as an enemy.
Sedinam K.C. Moyowasifza-Curry, is one of four officially recognized candidates, in addition to Stein, seeking the Green Party nomination for President. http://www.gp.org/the_green_party_recognizes_five_candidates_for_the_green_presidential_nomination She told this correspondent, "f or the past few weeks polls have shown widespread dissatisfaction with the presumed Democratic and Republican nominees and a desire for more options on the ballot, especially among young voters." http://thehill.com/blogs/ballot-box/presidential-races/280329-poll-voters-want-independent-candidate-over-clinton-trump
" As our campaign has traveled around our great nation, we have spoken directly with the Democratic membership base, and activists, that are awake to the corruption in their own corporate controlled political party and our flawed election sy stem. 44 percent of American voters have registered as Independent. We are reaching out to them too ," said Moyowasifza-Curry.
Moyowasifza-Curry also believes that the Green Party can become a home for people of color. This has not been a strength, to say the least. In order to do so, "Greens need to adopt a strategy with specifics o n how to end White p rivilege and White s upremacy within our national, state and local chapters", she said. She has proposed a detailed strategy and time line for doing so. "In ending White privilege and White supremacy, we must also come to the understanding that t here are no races, only one human species. One example of how this strategy is already being implemented is the Maine Green Party's discussion on its Facebook page," she said.
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