He should hit continue to hit her on Goldman, but expand the territory to the other donors such as CCA, JP Morgan, Citigroup ect, the dark money her Clinton Foundation has taken in, the weapons that foundation sold in destitute third world countries in order to fund factional strife, the 'donations' taking from big fracking firms and what she did as Secretary of State to push the fracking initiative to many global arenas just by nature of the lobby ect
Just confining the argument to Goldman is ineffective, because he is not so much taking on Clinton, but the entire corrupt system that she represents, its not just confined to one bank like Goldman or even one special interest like WallStreet or the banking sector. She is hardly alone in the GOP or the Democratic parties when it comes to 'favors', hands outs and corrupt connections, but she is the biggest name example.
You can tell Bernie is not a democrat, and the other Democrats are absolutely correct to hit him on being a DINO, but only in the sense that he has never been comfortable with kowtowing to a party affiliation over pointing out inherent systemic flaws.
The other Dems can hide behind the DNC and their political affiliations, but the Bern cannot. That might hurt him significantly if his message is not strong enough.
