IMO -- just a really crappy candidate in year where that will actually matter. So where their voters go matter, but if their voters were full fledged lefty loons they'd already support Sanders IMO, so maybe their voters do go to Biden. I think - and hope - that it's Biden, and I can see him choosing either Harris or Mayor Pete as his VP depending on where he gets hit harder from the left: if it's identity politics stuff, Harris is the obvious pick as a minority woman whereas if he's just seen as overall not progressive or exciting enough then I'd probably pick Mayor Pete or Beto.