“There’s a growing realization that Sanders could end up winning this thing, or certainly that he stays in so long that he damages the actual winner,” said David Brock, the liberal organizer, who said he has had discussions with other operatives about an anti-Sanders campaign and believes it should commence “sooner rather than later.”

But to some veterans of the still-raw 2016 primary, a heavy-handed intervention may only embolden him and his fervent supporters.

R.T. Rybak, the former Minneapolis mayor who was vice chairman of the Democratic National Committee in 2016, complained bitterly about the party’s tilt toward Mrs. Clinton back then, and warned that it would backfire if his fellow mainstream Democrats “start with the idea that you’re trying to stop somebody.”

If the party fractures again, “or if we even have anybody raising an eyebrow of ‘I’m not happy about this,’ we’re going to lose and they’ll have this loss on their hands,” Mr. Rybak said of the anti-Sanders forces, pleading with them to not make him “a martyr.”