Dirty Harry wants the 500 Superdelegates to pick the winner.
I think this has more to do with the "down ticket" than with any hope of winning the presidency. (The concern here isn't the presidency; it's the Senate, where the Dems are absolutely desperate to get a handle on the judicial appointments (in particular, the next SCOTUS appointment) being made by Trump and moved forward by McConnell.)
I think if a "crazy" wins the nomination, the party knows that many moderate Dems will stay home and they lose their chances to take control of the Senate. They will gladly toss away a shot at the presidency (because it was always going to be a long shot anyway) for the sake of gaining control of the Senate.
Prior to 1948, it required a 2/3 supermajority at the Democratic National Convention to get a nominee. It required a 2/3 vote to amend that rule, but they got the required 2/3 vote that year to reduce the requirement for a nomination down to simple majority, and it has been that way ever since.
Due process would require a procedural majority vote to reduce the needed vote for nomination down to plurality.
Interestingly, it was Reid himself who threw due process out the window in regard to Senate confirmations. Prior to Reid, filibusters extended to confirmations as well as legislation. It should have required 60 votes to separate confirmations from legislation, but Reid deemed that it required a majority (I think that’s right - maybe he did it by fiat).