Elizabeth Warren and Pete Buttigieg have been sparring over their work histories and health-care policies in recent weeks, but one Democratic 2020 candidate who poses a major threat to both of them is for now going mostly unchallenged: Bernie Sanders.

Aides to some of the Vermont senator’s competitors said he is getting a pass for a reason. Democratic candidates think Mr. Sanders’s core supporters are unlikely to leave his side, so there is no point trying to pick off that support, the aides said. But they also don’t think...