Sanders' line, "I do not have a super PAC, and I do not want a super PAC" is correct with the right terminology. He hasn't tacitly sanctioned a SuperPAC and there is no affiliation or coordination between the Nurses' one and his campaign. That's the difference, even though it's still an outside group contributing to the nomination/election of Sanders.
Not to mention most people inclined to support Sanders would see a very relevant difference between a superPAC funded by wealthy donors and one funded by a labor union. I mean come on.
Couple of reasons:
1. Sanders hasn't solicited donations to the PAC
2. The PAC is run by a union, which you must realize is ideologically different than big donors
3. The PAC doesn't accept money from donors, it is run by union dues
4. The PAC has spent a fraction of what has been spent by the other PACs, which is extremely relevant in terms of the influence they've held this election cycle.
5. Bernie Sanders has had no correspondence with the PAC, which cannot credibly be said for the other candidates' PACs
6. PACs are definitely not SuperPACs. They were allowed before Citizen's United, and you can give a maximum of $5000 to a PAC in a year, which is less than you can give to a candidate (primary + general election), and much less than the $8 million that George Soros just gave Hillary's SuperPACs.
