With Mr. Sanders favored to claim another victory in the Nevada caucuses on Saturday, some Democrats are frantically searching for a piece of kryptonite.

A Democratic group aimed at promoting moderates began airing digital ads this week that attack the costs of Mr. Sanders’s proposals. The group’s efforts follow a similar advertising campaign, bankrolled by a different Democratic organization, that questions whether Mr. Sanders can beat Mr. Trump in November.

In recent days, Michael R. Bloomberg’s team has shifted its strategy from largely ignoring Mr. Sanders in favor of focusing on Mr. Trump to targeting their primary rival more aggressively than anyone else in the field. At Wednesday night’s debate in Las Vegas, rivals hit him harder than ever before on his policies, the vitriolic tone of some of his supporters, his defense of socialism, and his health. There’s also some speculation among Democratic officials about stopping Mr. Sanders at the party convention in July.

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.”

Throughout the primary race, as soon as someone has risen to the top of the pack, attacks have piled on. Senator Elizabeth Warren of Massachusetts faced a barrage of questions about her “Medicare for all” plan after a summer surge in the polls. Former Mayor Pete Buttigieg of South Bend, Ind., was the subject of attacks after his poll numbers rose in Iowa.