After a campaign trail heart attack this fall, Bernie Sanders’ 2020 bid seemed to be on the ropes. Sanders had been losing steam even before his medical emergency, but with questions about his health alongside longstanding concerns over electability, the senator’s White House prospects were suddenly in doubt.

Remarkably, Sanders not only pulled through, but his campaign has thrived in the months since. In polls, Sanders has leapfrogged over Elizabeth Warren, who at one time outstripped him, and currently sits second to Joe Biden in national polls. While he remains a distant third in South Carolina, he’s leading in the early primary state of New Hampshire and is currently a close second to Pete Buttigieg in Iowa, where the first contest of the primary is under two months away.

In a sign of his increasing viability, Sanders appears to have captured the interested of the Democratic establishment, where he played antagonist in 2016. According to Politico, there’s been a newfound interest in Sanders in Democratic conclaves over the past few weeks. “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,” David Brock, a longtime Clinton ally, told the outlet. “They’ve both proven to be very resilient.”

Dan Pfeiffer, a former adviser to Barack Obama, added that “people should take [Sanders] 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.”

One reason for the fresh look at Sanders, sources told the outlet, is Warren’s plight. Though Sanders entered the 2020 race with a massive progressive movement behind him, Warren seemed to co-opt some of that momentum, rising steadily in polling over the summer and into the fall. As Sanders has risen, however, Warren has faltered—her tiff with Pete Buttigieg in the most recent Democratic debate netted out more or less evenly, and on Friday CNBC reported that her Q4 fundraising had slowed significantly. She finished Q3 with $24.6 million, per her campaign; in Q4, which ends in four days, she’s brought in $17 million. That’s coincided with a slip in the polls. Where she was once on par with Biden, even outpacing him in some polling, she’s now well behind him once again. (A spokesperson for Warren did not return CNBC’s request for comment.)

That’s not to say Sanders and Warren are exact counterbalances for each other, or that there isn’t still some skepticism of Sanders from within the establishment, particularly over his support for Medicare for All. Still, serious consideration from party insiders—and a whittling of options—could cement Sanders as the progressive counterweight to Biden. “A quarter of the electorate will potentially go for him,’” ex-Buttigieg campaign staffer Michael Ceraso, who worked for Sanders in 2016, told Politico. Sanders’ support “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.’”

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