This is what makes Trump so scary in a lot of ways, but specifically because of how good his instincts in Politics is. Pointing this out is essentially him winning the general election UNLESS its vs Bernie, which I've already stated is the only chance we have of beating Trump. Its a genius play by him and its not the first time he's pulled out this card in 2020. If they cheat Bernie then Trump wins.
For those that don't understand where I'm coming from, let's assume Bernie gets cheated and that for some reason Bernie supporters who just got cheated don't actually do the very likely thing of taking out their anger right there at the convention (Most of the people I follow on twitter are heading to the convention). They get bloomberg and then Trump keeps hammering away at the cheating of Bernie. Bernie supporters are angry, we're most likely going in with the most delegates and getting robbed so they say screw this. Some will see this and decide to screw the dem party by voting trump, while the majority will stay home and any chance of them coming out with trump hitting this message causing them to remain angry lessens the chances of them coming out to vote for the dem nominee. He doesn't need to convert anyone, he just needs to suppress the vote and that's why this is brilliant.
The funniest part is, this wont only turn off Bernie supporters and real lefties, but it will also disgust people watching the nomination process, regular folks who aren't hyper political who saw Bernie get the most delegates just to get screwed by the DNC and turn them way off of whoever the DNC anoints.
Again, no one but Bernie can beat Trump in 2020 and even then it will be difficult because Trump's political instincts are scarily good and honestly, he'd be better off without advisors. But then again, this is pretty much what I think the Dems are after somewhat. They'd prefer to lose to Trump than win with Bernie.