Screaming and shouting are a distinct possibility from the Sanders camp at the event, he says.

McArthur and other Sanders supporters are approaching the gathering with the enthusiasm that has powered the effort from the start — holding garage sales, delivering pizza and raising money online to pay for their travel to Philadelphia.

But their nerves are raw now over the Democratic Party's perceived slights against the insurgent candidate and they are clinging to a bygone hope that Sanders can wrest the nomination from Hillary Clinton despite her overpowering lead in delegates.

As these super-fans chant "Bernie or bust," Democratic officials are growing increasingly worried about dissent, especially after a recent state convention in Nevada turned raucous. Some of the Sanders backers who are going to the convention as delegates for him — and there are more than 1,400 — give party officials little reason for comfort.