I don't blame Democrats who had a problem with Sanders because he refused to align as a Democrat. There's a certain arrogance in spending half your energy, during your career, attacking the Democratic Party, and then asking the Democrats to support your presidential ambitions. But, in the end, what I care about is beating Trump. If Sanders provides the best hope of that, I'll support him enthusiastically, without requiring that he become a "full-fledged" Democrat. He seems like a good man, with decent policy ideas, and more to the point he'd be vastly better than Trump.
One just has to wonder what the difficulty is with Sanders not switching officially to the Democratic Party if he wants so badly to run in it? One would think he could just become a registered Democrat and that would pretty much be it. It would show all of America that he is now truly serious and aligned with the party that he wants to run in. But that hesitancy on his part to do so continues to be problematic. Why do that? You either like the political party enough or you don't. And if you don't you don't join it officially. The problem lies with those who just want to run as Democrats but don't want to really give up their own political party or leanings.
The best policy is of course to make sure that whomever wants to solicit their help in running in the Democratic Party is first and foremost REGISTERED as a Democrat. That should ALWAYS be the Cardinal Rule! No exceptions should be accepted or allowed.