For a party obsessed with learning from the mistakes of 2016, when the sheer improbability of Mr. Trump’s winning the White House dominated the discourse, the disbelief in the potential of Mr. Sanders to win the nomination and the reluctance of candidates to challenge him mystifies some Democrats — even those working for him. And the revelation that Russia may be trying to help him, four years after it helped Mr. Trump, only confuses matters more, because it is far from clear how that foreign interference will affect the race.

For months, top Sanders aides feared that a rival, perhaps Mr. Bloomberg, would start an anti-Sanders super PAC. That kind of broad, well-funded effort has not materialized. And of all the candidates, only Tom Steyer has released an ad attacking Mr. Sanders, targeting his failure to put a price tag on his Medicare for All Plan.

“A lot of it is the same thing you saw happening in the Republican establishment four years ago,” said Matt Bennett, a founder of the moderate think tank Third Way and a vocal critic of Mr. Sanders. “Suddenly in April they woke up and realized that Donald Trump could actually win.”