Are you referring to the tweet where Bernie just says, based on their own words, the third way group says anybody but Bernie? Because if that is too much of 'an attack' for the democratic primary, the entire field has done just as bad if not worse towards fellow Democrats.
It alienates Warren supporters (as Sanders supporters are want to do) and presents to his base that they shouldn't even think about supporting anyone else in the primary.
So then if it ultimately either comes down to Warren vs Biden, or Sanders vs Biden, in scenario 1 your Sanders supporters aren't going to jump ship for Warren the 'corporatist'. And in scenario 2 Warren supporters will jump ship to elsewhere because Sanders has alienated them. Then we get Joe Biden as a candidate over either of them, and both of them would be better than Biden. Because Biden is an actual corporatist who has very little progressive credentials. Then once again the centrists jump and dance as they get their preferred candidate and once again force us to vote for someone who won't enact change or ultimately get someone who will change things for the much worse (Donald J. Trump)
I know of no one who likes Bernie and doesnt think Warren is pretty good on the left. This is such an extreme minority. ---