WASHINGTON — When Leah Daughtry, a former Democratic Party official, addressed a closed-door gathering of about 100 wealthy liberal donors in San Francisco last month, all it took was a review of the 2020 primary rules to throw a scare in them.
Democrats are likely to go into their convention next summer without having settled on a presidential nominee, said Ms. Daughtry, who ran her party’s conventions in 2008 and 2016, the last two times the nomination was contested. And Senator Bernie Sanders is well positioned to be one of the last candidates standing, she noted.
“I think I freaked them out,” Ms. Daughtry recalled with a chuckle, an assessment that was confirmed by three other attendees. They are hardly alone.
From canapé-filled fund-raisers on the coasts to the cloakrooms of Washington, mainstream Democrats are increasingly worried that their effort to defeat President Trump in 2020 could be complicated by Mr. Sanders, in a political scenario all too reminiscent of how Mr. Trump himself seized the Republican nomination in 2016.
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How, some Democrats are beginning to ask, do they thwart a 70-something candidate from outside the party structure who is immune to intimidation or incentive and wields support from an unwavering base, without simply reinforcing his “the establishment is out to get me’’ message — the same grievance Mr. Trump used to great effect?
But stopping Mr. Sanders, or at least preventing a contentious convention, could prove difficult for Democrats.
He has enormous financial advantages — already substantially outraising his Democratic rivals — that can sustain a major campaign through the primaries. And he is well-positioned to benefit from a historically large field of candidates that would splinter the vote: If he wins a substantial number of primaries and caucuses and comes in second in others, thanks to his deeply loyal base of voters across many states, he would pick up formidable numbers of delegates for the nomination.
That prospect is not only spooking establishment-aligned Democrats, but it is also creating tensions about what, if anything, should be done to halt Mr. Sanders.
Some in the party still harbor anger toward over the 2016 race, when he ran against Hillary Clinton, and his ongoing resistance to becoming a Democrat. But his critics are chiefly motivated by a fear that nominating an avowed socialist would all but ensure Mr. Trump a second term.
“There’s a growing realization that Sanders could end up winning this thing, or certainly that he stays in so long that he damages the actual winner,” said David Brock, the liberal organizer, who said he has had discussions with other operatives about an anti-Sanders campaign and believes it should commence “sooner rather than later.”
But to some veterans of the still-raw 2016 primary, a heavy-handed intervention may only embolden him and his fervent supporters.
R.T. Rybak, the former Minneapolis mayor who was vice chairman of the Democratic National Committee in 2016, complained bitterly about the party’s tilt toward Mrs. Clinton back then, and warned that it would backfire if his fellow mainstream Democrats “start with the idea that you’re trying to stop somebody.”
If the party fractures again, “or if we even have anybody raising an eyebrow of ‘I’m not happy about this,’ we’re going to lose and they’ll have this loss on their hands,” Mr. Rybak said of the anti-Sanders forces, pleading with them to not make him “a martyr.”
To a not-insignificant number of Democrats, of course, Mr. Sanders’s populist agenda — including “Medicare for all” and a focus on working families — is exactly what the country needs. And he has proved his mettle, having emerged from the margins to mount a surprisingly strong challenge to Mrs. Clinton, earning 13 million votes and capturing 23 primaries or caucuses.
The good news for Mr. Sanders’s foes is that his polling is down significantly in early-nominating states from 2016, he is viewed more negatively among Democrats than many of his top rivals, and he has already publicly vowed to support the party’s nominee if he falls short.
”Bernie Sanders believes the most critical mission we have before us is to defeat Donald Trump,” said Faiz Shakir, Mr. Sanders’s campaign manager. “Any and all decisions over the coming year will emanate from that key goal.”
Or, as former Senator Claire McCaskill put it: “One thing we have now that we didn’t in ’16 is the uniting force of Trump. There will be tremendous pressure on Bernie and his followers to fall in line because of what Trump represents.”
But Mr. Sanders is also taking steps that signal he is committed for the duration of the race — and will strike back aggressively when he’s attacked. On Saturday his campaign sent a blistering letter to the Center for American Progress, a Clinton-aligned liberal think tank, accusing them of abetting Mr. Trump’s attacks, of playing a “destructive” role in Democratic politics, and of being beholden to “the corporate money” they receive. The letter came days after a website aligned with the center aired a video highlighting Mr. Sanders’s status as a millionaire.
With other mainline party leaders, he is offering more honey than vinegar.
Last month, for example, he used his first trip to Iowa as a 2020 candidate to quietly meet with Jeff Link, a veteran party strategist, and Patty Judge, the former state agriculture secretary, to discuss rural policy and politics, according to a Democrat familiar with the meeting. Mr. Sanders’s campaign also reached out to Randi Weingarten, the head of the American Federation of Teachers and a top Clinton ally in 2016, to have her join them at what they dubbed an “Ohio workers town hall” on Sunday.
“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 is discussed “endlessly” in his orbit, and among Democratic leaders it is 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 the former New York mayor Michael R. Bloomberg, 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; Senator Chuck Schumer, the majority 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.
“He did us a disservice in the last election,” said Mr. Schwartz, a longtime Clinton supporter who said he will support former Vice President Joseph R. Biden Jr. in this primary.
But it is hardly only Mr. Sanders’s critics who believe the structure of this race could lead to a 50-state contest (like the previous two Democratic primaries) and require deal-cutting to determine a nominee before or at the convention (unlike the previous two Democratic primaries).
“If I had to bet today, we’ll get to Milwaukee and not have a nominee,” said Ms. Daughtry, who was neutral in the 2016 primary.
The reason, she theorized, is simple: Super Tuesday, when at least 10 states vote, comes just three days after the last of four early states. After that, nearly 40 percent of the delegates will have been distributed — and, she suspects, carved up among Democratic candidates so that nobody can emerge with a majority.
Unlike Republicans, who used a winner-take-all primary format, Democrats use a proportional system, so candidates only need to garner 15 percent of the vote in a primary or caucus to pick up delegates. And even if a candidate fails to capture 15 percent statewide, he or she could still win delegates by meeting that vote threshold in individual congressional districts.
Should no bargain be struck by the time of the first roll call vote at the 2020 convention in Milwaukee — such as a unity ticket between a pair of the leading delegate-winners — the nomination battle would move to a second ballot. And under the new rules crafted by the D.N.C. after the 2016 race, that is when the party insiders and elected officials known as superdelegates would be able to cast a binding vote.
The specter of superdelegates deciding the nomination, particularly if Mr. Sanders is a finalist, is highly unappetizing to party officials.
“If we have a role, so be it, but I’d much prefer that it be decided in the first round, just from a unity standpoint,” said Senator Debbie Stabenow of Michigan.
That may not happen should Mr. Sanders, sustained by his online fund-raising network, remain in the primary but fail to win a majority of delegates after the last states vote in June. If he is unable, for example, to broaden his appeal with nonwhite voters, he could accumulate hundreds of delegates but still fall short of the nomination.
Yet that result might be not fully realized until later in the primary calendar — well after Mr. Sanders has put his money to work.
“If he is consistently raising $6 million more than his next closest opponent, he’s going to have a massive financial advantage,” said Rufus Gifford, former President Barack Obama’s 2012 finance director, noting that Mr. Sanders will be able to blanket expensive and delegate-rich Super Tuesday states like California and Texas with ads during early voting there.
Mr. Gifford, who has gone public in recent days with his dismay over major Democratic fund-raisers remaining on the sidelines, said of Mr. Sanders, “I feel like everything we are doing is playing into his hands.”
But the peril of rallying the party’s elite donor class against a candidate whose entire public life has been organized around confronting concentrated wealth is self-evident: Mr. Sanders would gleefully seize on any Stop Bernie effort.
“You can see him reading the headlines now,” Mr. Brock mused: “‘Rich people don’t like me.’”