Sen. Bernie Sanders' recent surge in national and early-state polls, enthusiastic progressive base, and resilience in the aftermath of his heart attack have reportedly forced some within the Democratic establishment who were previously dismissive of the Vermont senator to concede—both in private and in public—that he could ultimately run away with the party's presidential nomination.

"For months the Vermont senator was written off by Democratic Party insiders as a candidate with a committed but ultimately narrow base who was too far left to win the primary," Politico reported Thursday. "But in the past few weeks, something has changed. In private conversations and on social media, Democratic officials, political operatives, and pundits are reconsidering Sanders' chances."

"It is harder and harder to ignore him when he's rising in every average that you see."

—Faiz Shakir, campaign manager for Sen. Bernie Sanders

David Brock, a Democratic operative and long-time ally of Hillary Clinton who earlier this year discussed launching an "anti-Sanders campaign," told Politico that Sanders and former Vice President Joe Biden have "both proven to be very resilient."

"It may have been inevitable that eventually you would have two candidates representing each side of the ideological divide in the party," Brock added. "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."

Former Obama adviser Dan Pfeiffer said "people should take him very seriously," referring to Sanders.

"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 a adviser to former President Barack Obama. "He could build a real head of steam heading into South Carolina and Super Tuesday."

According to Real Clear Politics polling averages, Sanders is in a close second place in Iowa, first place in New Hampshire, and virtually tied with Sen. Elizabeth Warren (D-Mass.) for second in Nevada. A poll last week from the UC Berkeley Institute of Government Studies showed Sanders leading the Democratic field in California, a crucial Super Tuesday state.

Sanders' polling strength, combined with the collapse of "other candidates with once-high expectations, such as Kamala Harris, Cory Booker, and Beto O'Rourke," has begun to change the minds of Democratic insiders, according to Politico.

Faiz Shakir, Sanders' campaign manager, said figures within the Democratic establishment are not rethinking Sanders' chances to win the nomination "out of the goodness of their heart."

They are doing so, Shakir said, because "it is harder and harder to ignore him when he's rising in every average that you see."

Shakir went on to tell Politico that the campaign is eager to have a conversation about Sanders' electability.

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

While Sanders has received significantly less attention in the corporate media than his Democratic rivals, Politico noted that 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," David Axelrod, a former Obama adviser, said on CNN following the debate. "He's actually picked up. And the fact is Bernie Sanders is as consistent as consistent can be."

Sanders' campaign announced following the debate in Los Angeles that it raised more than a million dollars on debate day from tens of thousands of individual contributions.

"The fundraising total and number of individual donations," the campaign said in a statement, "was the highest for Sanders' campaign during a debate day in 2019, which is just the latest sign of the momentum his campaign is seeing all over the country."