I don't think Bernie wants to divide the party. I think he really wanted to be nominee because I think he genuinely hates Trump and wanted to defeat him in November. While it stings him that more voters aren't seeing him as the person to achieve that, I think he genuinely wants whoever is nominee to destroy Trump. To that end, when he warns Biden that he needs to acknowledge the progressives who supported Bernie (as well as those progressives who compromised to support Biden out of a fear of Trump winning), he isn't threatening to keep those voters from supporting Biden, he is warning that even though he will support Biden, he can't control how his supporters will choose to vote in November. A genuine reach out effort to those progressive voters by Biden will mean more of them are likely to unify behind Joe. That is what Bernie wants and it sucks people are willing to blame him if a fraction of his supporters don't get behind Joe.
So it is disappointing that Bernie's warnings to Biden and the centrists are being spun as an attempt to divide. Remember, he practically begged his supporters to get behind Clinton in 2016. He isn't trying to fracture the democratic party, he's trying to save it so it can save America from Trump. For all Biden's and the moderates' lip service about compromising, I think Bernie is actually showing he is willing to compromise to stop Trump. I hope Joe will take this seriously and try to include the left of the party in his movement. He has to realize that a lot of the people who voted him to victory in the primaries have been progressives willing to compromise their own values if it means stopping trump. So let's see him meet those voters at least halfway. They gave him a lot and he needs to repay the favor to them and not just to his centrist base.
Despite my earlier statements, I am considering holding my nose and supporting Biden with a vote in November. It will depend on whether his attempts to reach out to and compromise with the progressives will be genuine or lip service. Too many democratic candidates in the last 40 years have worried about reaching out to moderate republicans that I feel they've lost sight of the core democratic values that defined the party's platform from the thirties to the early seventies. They became decaff republicans. At this point, they need to stop worrying about those mythical on-the-fence republicans and start unifying the actual democrats who lost faith in the party leadership after decades of being all but ignored. They are more than just a few jaded millennial Bernie bros. Don't write them off again, Joe.