The Democratic Electorate is determined to find a candidate that can defeat Donald Trump.
To them, whether that candidate agrees with them on the issues is secondary because if you can't defeat Trump, they won't chose you.
Joe Biden is determined to instil into the minds of the electorate that he can defeat Trump and Bernie can't.
That and the fact that he's Obama's VP is why the Democratic Electorate wants to vote for him.
This is where Bernie needs to make that argument.
1. Why he is more electable than Joe Biden.
2. Why he can defeat Trump.
3. Why Joe Biden CANNOT defeat Donald Trump.
This is where Bernie HAS to go scorched Earth on Joe Biden.
Oh and one more thing...
ON TOP OF THAT, HE NEEDS TO HIT BIDEN OVER THE HEAD FOR HIS COMMENTS ON HIM VETOING MEDICARE FOR ALL.
Not only that but...
For example:
Bernie should say 'That assertion was made by the same intelligence community that stages violent coups in Latin American countries and lied about Saddam Hussein having Weapons of Mass Destruction'
Bernie should say 'How are we gonna pay for our current heath care system that is even more expensive than Medicare for All yet still leaves millions of people uninsured or underinsured'
Bernie should say 'No I'm representing the ideals of Barack Obama, Franklin Delano Roosevelt, John F Kennedy, and Eisenhower that got them elected in the first place'