“If anybody thinks Bernie Sanders is incapable of doing politics, they haven’t seen him in Congress for 30 years,” said Tad Devine, Mr. Sanders’s longtime strategist, who is not working for his campaign this year. “The guy is trying to win this time.”
But such outreach matters little to many Democrats, especially donors and party officials, who are growing more alarmed about Mr. Sanders’s candidacy.
Mr. Brock, who supported Mrs. Clinton’s past presidential bids, said “the Bernie question comes up in every fund-raising meeting I do.” Steven Rattner, a major Democratic Party donor, said the topic was discussed “endlessly” in his orbit, and among Democratic leaders it was becoming hard to block out.
“It has gone from being a low hum to a rumble,” said Susan Swecker, the chairwoman of Virginia’s Democratic Party.
Howard Wolfson, who spent months immersed in Democratic polling and focus groups on behalf of former Mayor Michael R. Bloomberg of New York, had a blunt message for Sanders skeptics: “People underestimate the possibility of him becoming the nominee at their own peril.”
The discussion about Mr. Sanders has to date been largely confined to private settings because — like establishment Republicans in 2016 — Democrats are uneasy about elevating him or alienating his supporters.
The matter of What To Do About Bernie and the larger imperative of party unity has, for example, hovered over a series of previously undisclosed Democratic dinners in New York and Washington organized by the longtime party financier Bernard Schwartz. The gatherings have included scores from the moderate or center-left wing of the party, including Speaker Nancy Pelosi of California; Senator Chuck Schumer of New York, the minority leader; former Gov. Terry McAuliffe of Virginia; Mayor Pete Buttigieg of South Bend, Ind., himself a presidential candidate; and the president of the Center for American Progress, Neera Tanden.