This group, which differs from more traditional Democratic activists and party operatives, was sure that Mr. Mueller would implicate the president in conspiracies to influence the 2016 election and obstruct justice. That belief drove an almost obsessive Mueller fandom among the group, which formed its own internet subculture among the wider anti-Trump base. Mr. Mueller was personally revered for being everything the group thought Mr. Trump was not, in ideology, integrity and temperament.

A popular podcast called “Mueller, She Wrote,” which is streamed up to 200,000 times a week, calls its Russia-obsessed fans Muellerites. One of the three hosts, a California comedian who goes by A. G. and has a tattoo of Mr. Mueller’s silhouette on her arm, said the community was in what could only be described as a collective state of muted shock.

“I’ve had to talk a couple people off the ledge,” she said.

One fan of the podcast, Anne Craig-Tillmond, said she wants to see the full report before she’s “completely deflated.”

“I’ve been emotionally invested from the start, and was I let down by what Barr said? Absolutely,” Ms. Craig-Tillmond said. “Was I let down by what Mueller said? I don’t know what he said.”

The Facebook page of the Robert Mueller Fan Club says on its “About” page, “All hail Robert Mueller — through his work we shall be delivered from this ignorance and evil we suffer.” Barbara Llewellyn, a retired real estate agent from Naperville, Ill., had written “Hallelujah” on the page. But by Monday, she had changed her tune.

“I am very disappointed, because I felt that there were so many public instances where Trump acted guilty and tried to shut down the investigation in various ways,” Ms. Llewellyn said. “And I am realizing that this is just Barr’s interpretation of what the report said. And I’m hoping very quickly that Congress gets ahold of it totally unredacted, and we can go from there.”

Some progressive activists have been calling for Mr. Trump’s impeachment almost since he took office, and billionaire Tom Steyer ran television advertisements with that aim across the country. But the sense of profound disappointment that has set in on the left illustrates why Democratic campaigns have been so leery of resting their electoral hopes on Mr. Mueller’s investigation and the possibility of impeachment.