Politico reported that they obtained a script which the Bernie campaign has distributed to canvassers and volunteers. What was reported in the article, since the entire script wasn't posted, seems to be the worst parts of the script; however, they aren't aggressive at all, and in one case very positive of Sen. Warren.
The reference to the script, which is implied to be an attack, is that her supporters are "highly-educated, more affluent people who are going to show up and vote Democratic no matter what” and how Warren isn't bringing any "new bases into the Democratic Party.” Personally, I don't see how this can be seen of as attacking another candidate, when its simply going based off polling, where Sen. Warren does better with people with higher than a Bachelor's Degree, and have higher household incomes than Sen. Sanders does. Sen. Warren's supporters are on Democratic lock-down, with the #VoteBlueNoMatterWho support of the Democratic Party. It isn't attacking if Sanders' campaign pushes this, it is simply differentiating, which is what primaries are for.
The other reference to the script was optional and was, “I like Elizabeth Warren. In fact, she’s my second choice. But here’s my concern about her...” which then gave a list of supposed "criticisms of Warren," but those criticisms weren't published by Politico unfortunately. Again, I don't see how this is a criticism. Literally saying "I like Elizabeth Warren" and that she is the volunteers "second choice," how is that attacking her? It isn't, it is differentiating, again which is what primaries are for.
Sen. Warren has responded to the reporting by Politico claiming that Sen. Sanders "is sending his volunteers out to trash me," which if she thinks that, this tepid differentiating between the two candidates while canvasing, is trashing her, then I don't see how she can be prepared for a general election with Donald Trump.
She goes on to say that we "need to unite our party," which is an establishment talking point used against non-establishment candidates, like Sen. Sanders, when in recent history, it is the establishment candidates supporters which are more likely to switch parties, like with Hillary Clinton's 2008 campaign, which spawned the PUMAs, or Party Unity My Ass group of her supporters, which turned out to be a decent portion, with nearly 25% of polled Hillary voters in the general election in 2008 ended up voting for Sen. McCain, not Sen. Obama. We all can see that 25% of Hillary to McCain voters is much larger than the 12% Bernie to Trump voters in 2016.
This will most likely be brought up at the January 14th Democratic Debate, leading up to the Iowa Caucus, February 3rd, to try to get Sen. Sanders and Sen. Warren attacking each other on stage; however, knowing Sen. Sanders' style he won't attack unless he is attacked, though the caucuses only being 3 weeks away, Sen. Sanders may try a different strategy.
Sen. Sanders is headed into this Iowa debate with 20% support, followed by Sen. Warren at 17%, according to a Des Moines Register/CNN poll.