After his 2016 Democratic primary loss to Hillary Clinton, many thought Bernie Sanders had hit his political high water mark. Now, they're venting frustration over his persistence and doggedly loyal base.

The Vermont senator's 2020 White House bid continues to poll well nationally and in the four states hosting the opening primary contests, starting with Iowa on Feb. 3.

The strong performance by Sanders, 78, in early polls had been attributed to residual name recognition carried over from his run against Clinton, the former secretary of state, senator, and first lady who went on to lose to President Trump that fall.

Sanders is competitive against 2020 Democratic rivals Joe Biden, Pete Buttigieg, and Elizabeth Warren, and his performance in recent public opinion surveys and strong fundraising numbers are escalating tensions within the party, with many members still reluctant to back Sanders, a longtime independent and socialist who suffered a heart attack in early October, as this cycle's nominee.

Yet Establishment complaints may just have the opposite result of their intended effect, energizing legions of Sanders fans.

Clinton herself kicked off the Sanders snubs, declining to say in an interview if she would back him if he wins the 2020 primary.

“I'm not going to go there yet. We're still in a very vigorous primary season,” she told the Hollywood Reporter.

A day-and-a-half earlier, the New York Times editorial board backed Warren and Minnesota Sen. Amy Klobuchar over Sanders, who was mayor of Burlington, Vermont, from 1981 to 1989, then an independent House member for 16 years before joining the Senate in 2007.

The newspaper’s board acknowledged Sanders’s influence on the crowded field, helping once seemingly radical and fringe ideas, such as universal healthcare, free college, a higher minimum age, and paid family leave, find acceptance in mainstream Democratic politics. It admitted his criticism of the party's catering to “rich donors” over the middle class were fair and that he brought new voters into the fold. Yet, the board still had doubts about his health after he suffered a heart attack in October and, more broadly, his approach to politics.

“He boasts that compromise is anathema to him. Only his prescriptions can be the right ones, even though most are overly rigid, untested and divisive. He promises that once in office, a groundswell of support will emerge to push through his agenda. Three years into the Trump administration, we see little advantage to exchanging one over-promising, divisive figure in Washington for another,” it wrote.

And that is only the tip of the Democratic figure pile-on.

“If I were a campaign manager for Donald Trump, and I look at the field, I would very much want to run against Bernie Sanders,” Obama campaign manager Jim Messina said this month. “I think the contrast is the best. He can say, ‘I’m a business guy, the economy’s good, and this guy’s a socialist.’”

In addition, Sanders has tried to engage his opponents on the issues rather than through personal attacks, but his campaigns' swipes at competitors, and those of his ardent supporters, have rubbed some Democrats the wrong way. And they haven’t been afraid to call him out.

Sanders first distanced himself from a leaked campaign memo targeting Democrats leaning toward Warren , arguing she appealed only to affluent, well-educated members of the party who vote anyway, not expanding the base . The pair then broke a nonaggression pact to clash over their differing recollections of a private 2018 conversation in which Warren, 70, accused Sanders of questioning whether a woman could win the White House . Sanders has denied her claim, and CNN was trashed for ignoring his dismissal of the story when the topic was broached during last week’s Iowa debate . He then addressed gender in politics during an interview in New Hampshire over the weekend, reiterating how it was a “problem.”

Supporters of Sanders on the campaign trail and online haven’t shied away from confronting Biden, the former vice president on his Iraq vote or Buttigieg, the former South Bend, Indiana Mayor, over his sway with black Democrats . But the candidate himself was forced this week to apologize under pressure for an opinion piece published by a top backer who leveled unfounded corruption allegations against the 36-year Delaware senator. The article had been pushed by the Sanders team.

“It's genuinely depressing how the 2020 campaign is just a repeat of 2016. How many [times] was Hillary called corrupt? Thousands? Millions? Absolutely huge downside to these few weeks before Iowa is reliving the 2016 nightmare,” tweeted Neera Tanden, president of the Center for American Progress and a strident Clinton defender.

It's genuinely depressing how the 2020 campaign is just a repeat of 2016. How many was Hillary called corrupt? thousands? Millions? Absolutely huge downside to these few weeks before Iowa is reliving the 2016 nightmare. https://t.co/fONghIkBc6 — Neera Tanden (@neeratanden) January 20, 2020

Economics Nobel laureate and liberal New York Times opinion writer Paul Krugman added that the move was “a really bad look.”

“It illustrates everything that makes many Democrats distrust the Sanders team — and the buck stops with the candidate,” he wrote.

This is a really bad look. It illustrates everything that makes many Democrats distrust the Sanders team — and the buck stops with the candidate https://t.co/EY1QL7YbXx — Paul Krugman (@paulkrugman) January 20, 2020

But it remains to be seen whether Democratic Establishment pushback will steer the grassroots in another direction given a political environment in which the liberal wing is ascendant. Democrats who support Sanders, for example, have warned that many will stay home for the general election if a milquetoast candidate becomes the party’s next leader .

Fans such as Debbie Morrow, 68, told the Washington Examiner last fall that Sanders was “an inspiration” to the retiree from Spartanburg, South Carolina, because of his advocacy for "taking money out of politics and being more down to earth.”

Kilynn Lunsford, 38, a Philadelphia-based labor organizer working for Sanders, said after a rally in Davenport, Iowa, this month that he "galvanizes people" whenever they hear him speak.

"They understand that he's going to change this country for the better, and change the country for the working class, and they're not hearing that message from anyone else — ones that can actually hear that without all the media static manipulating what he's saying," she said.

Two weeks out from Iowa, Sanders currently trails Biden by an average of 8 percentage points nationwide, according to RealClearPolitics data. The senator averages 20.4% of the vote, to the former vice president's 28.4%.