“It may have been inevitable that eventually you would have two candidates representing each side of the ideological divide in the party. A lot of smart people I’ve talked to lately think there’s a very good chance those two end up being Biden and Sanders,” said David Brock, a longtime Hillary Clinton ally who founded a pro-Clinton super PAC in the 2016 campaign. “They’ve both proven to be very resilient.”

Democratic insiders said they are rethinking Sanders’ bid for a few reasons: First, Warren has recently fallen in national and early state surveys. Second, Sanders has withstood the ups and downs of the primary, including a heart attack. At the same time, other candidates with once-high expectations, such as Kamala Harris, Cory Booker and Beto O’Rourke, have dropped out or languished in single digits in the polls.

“I believe people should take him very seriously. He has a very good shot of winning Iowa, a very good shot of winning New Hampshire, and other than Joe Biden, the best shot of winning Nevada,” said Dan Pfeiffer, who served as an adviser to former President Barack Obama. “He could build a real head of steam heading into South Carolina and Super Tuesday.”

The durability of Sanders’ candidacy has come as a surprise even in some states where he performed strongly in 2016 and where he is attempting to improve his standing ahead of the 2020 election.

California state Sen. Scott Wiener, who defeated a Sanders-backed Democrat for his seat in the liberal-heavy San Francisco area in 2016, said Sanders has been “more resilient than I anticipated.”

“But in retrospect,” he added, “he has a very, very loyal following, and people have really stuck with him.”

Sanders is in second place in national polls, nearly 9 percentage points behind Biden, according to the most recent RealClearPolitics average. He is second in Iowa and first in New Hampshire. The latest CNN poll found he has the highest net favorability rating of any Democratic presidential candidate.

While Sanders’ supporters complain relentlessly that he has received less attention from the media than other candidates, he has also avoided sustained criticism that some of his rivals have suffered. That could be helping him, especially compared with Warren, who has recently come under fire from the left and center for her health care plan.

“If you really think about it, Bernie hasn’t been hit a lot with anything. It’s not like he’s getting hit by other campaigns,” said Michael Ceraso, a former New Hampshire director for Pete Buttigieg’s campaign who worked for Sanders in 2016.

“You sort of take for granted that he, like Biden, are institutional figures for very different reasons,” Ceraso said. “Early in the campaign, Bernie’s people said, ‘Look, this guy in these early states has a nice hold, and there’s a percentage of supporters, a quarter of the electorate will potentially go for him.’” He added, “It waned a little bit because people were looking at other options … and now they’re saying, ‘Wait a minute, this guy has been the most consistent of anyone.’”

At the beginning of the year — another high point for Sanders’ campaign, before Warren surged — some establishment Democrats talked about how to stop his momentum. Brock, who has a close relationship with many Democratic donors, said he has not heard anything like that in recent weeks: “That doesn’t mean it won’t happen. This is more of an analysis in the political world than in the donor world.”

Many moderate Democrats still dismiss Sanders’ candidacy. They believe his so-called ceiling remains intact and that Warren will depress any room for growth he might otherwise have.

“He can’t win the nomination,” said Matt Bennett, co-founder of the center-left group Third Way, adding that Sanders’ uptick is simply him “bouncing around between his ceiling and his floor a little bit more than people had thought he would.”

On the other hand, he acknowledged his staying power. “Not until the very end will people say to Bernie Sanders, ‘When are you dropping out?’”

A series of TV segments around last week’s Democratic debate illustrate the shift in how Sanders is being perceived. “We never talk about Bernie Sanders. He is actually doing pretty well in this polling,” former senior Obama adviser David Axelrod said on CNN after the event. “He’s actually picked up. And the fact is Bernie Sanders is as consistent as consistent can be.”

The same day on MSNBC, national political correspondent Steve Kornacki said, “Democratic voters like him, and if he starts winning, there could be a bandwagon effect.” GOP pollster Frank Luntz, who conducted a California focus group that found most participants thought Sanders had won the debate, said on CNBC, “I think you’re going to see continued movement. Sanders has been gaining in California over the past two months.”

Larry Cohen, chairman of the pro-Sanders group Our Revolution, said Warren’s candidacy is not a problem for Sanders if both of them can — together — amass a plurality of delegates heading into the convention.

“The math is that if you think of the voters for Warren and the voters for Sanders as two circles, yes, there is overlap, [but] most of the circles are separate,” Cohen said. “I think between them, we can get to a majority.”

If Sanders’ candidacy continues to be taken seriously, he will eventually be subjected to the scrutiny that Warren and Biden have faced for prolonged stretches. That includes an examination of his electability. “That conversation has never worked well for anyone,” Pfeiffer said.

Former California Gov. Gray Davis stopped short of saying firm support for “Medicare for All” would be an impediment for Democrats in the primary but suggested the risk for the nominee is significant.

“Californians and Americans, in general, like options — not mandates,” he said.

Faiz Shakir, Sanders’ campaign manager, said political insiders and pundits are rethinking his chances “not out of the goodness of their heart,” but because “it is harder and harder to ignore him when he’s rising in every average that you see.” And he welcomes a conversation about Sanders’ electability, he said.

“We want that,” he said. “I’d love to be able to argue why he stands a better chance to beat Donald Trump than Joe Biden.”

Christopher Cadelago contributed to this report.