Brittany Greeson for The New York Times
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.
To a not-insignificant number of Democrats, of course, Mr. Sanders’s populist agenda is exactly what the country needs. And he has proved his mettle, having emerged from the margins to mount a surprisingly strong challenge to Hillary Clinton, earning 13 million votes and capturing 23 primaries or caucuses.
His strength on the left gives him a real prospect of winning the Democratic nomination and could make him competitive for the presidency if his economic justice message resonates in the Midwest as much as Mr. Trump’s appeals to hard-edge nationalism did in 2016. And for many Sanders supporters, the anxieties of establishment Democrats are not a concern.
That prospect is spooking establishment-aligned Democrats, some of whom are worried that his nomination could lure a third-party centrist into the field. And it is also creating tensions about what, if anything, should be done to halt Mr. Sanders.
Some in the party still harbor anger over the 2016 race, when he ran against Mrs. 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.”
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…