– 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 Daughtry, who ran her party’s conventions in 2008 and 2016, the last two times the nomination was contested. And Sen. Bernie Sanders is well positioned to be one of the last candidates standing, she noted.

“I think I freaked them out,” Daughtry recalled with a chuckle, an assessment that was confirmed by three other attendees. They are hardly alone.

From canapé-filled fundraisers on the coasts to the cloakrooms of Washington, mainstream Democrats are increasingly worried that their effort to defeat President Donald Trump in 2020 could be complicated by Sanders, in a political scenario all too reminiscent of how Trump himself seized the Republican nomination in 2016.

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 Trump used to great effect?

But stopping 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.

To a not-insignificant number of Democrats, of course, Sanders’ 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 much as Trump’s appeals to nationalism did in 2016.

That prospect is spooking establishment-aligned Democrats, and creating tensions about what, if anything, should be done to halt Sanders.

Some in the party still harbor anger over the 2016 race, when he ran against 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 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.

Former Minneapolis Mayor R.T. Rybak, who was vice chairman of the Democratic National Committee in 2016, complained bitterly about the party’s tilt toward 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.”

The good news for Sanders’ foes is that his polling is down significantly in early-nominating states from 2016 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, Sanders’ campaign manager.