Wow he really is going after the Democratic establishment.
Sanders/Schweitzer third party perhaps?
I doubt Schweitzer would leave the party. But even if he wanted to, he would almost certainly run as a Democrat in the primaries first. You just don't get much media coverage as a 3rd party candidate. But being one of the only Dems to challenge Clinton as a Democrat will get you media coverage. Even if you're trailing in the polls by about 50%.
Heck, even Sanders commented that while he'd prefer to run as an Indy, he might run as a Dem just because it gets you in the nationally televised primary debates. If both Sanders and Schweitzer run as Democrats, the fact that there'll be two of them would also allow them to hold debates even if Clinton doesn't show up.