Three months ago, Sen. Bernie Sanders was back in Vermont, recovering from a heart attack and heart surgery that knocked him off the 2020 campaign trail and kicked off a tidal wave of questions about whether he could go forward in the Democratic primary, let alone win it.

Counted out by many after the health scare and overlooked for months while former mayor Pete Buttigieg, former Vice President Joe Biden and Sen. Elizabeth Warren dominated headlines, Sanders is now riding an uptick in polling and a new narrative around his campaign in the political mediasphere: Could Bernie Sanders really win this time?

The 78-year-old U.S. Senator from Vermont, a self-described democratic socialist who has pushed for a working-class revolution, is solidly second in most national polls behind Biden, and he either leads or is tied in recent polls in the first two states that vote, Iowa and New Hampshire.

Questions still persist whether Sanders, who fell short in the 2016 Democratic primary to Hillary Clinton, can expand and diversify his loyal base of young, predominantly white, progressives to secure the nomination.

But Sanders has demonstrated consistency while rivals have bounced around in polls, and his campaign says he's built to cross the finish-line. They point to stronger numbers among young black voters and Hispanic voters this time around and an ability to attract first-time primary voters.

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"I think the two people who will absolutely be in it until the end are Biden and Bernie," said Joe Trippi, a longtime Democratic campaign operative. "Does it turn into a Biden-Bernie race the way Clinton-Bernie did? Or is it three or four other people, billionaires, fighting it out until the end? I don't know."

A win in Iowa, where Sanders has steadily climbed in polls for weeks and has a robust organization, would give him considerable momentum heading into his neighboring New Hampshire, which he won four years ago, and Nevada, where he finished a close second behind Clinton.

He remains far behind in South Carolina, where Biden's strong support among African American voters has given him a commanding lead. But the Sanders team has pumped major resources into delegate-rich California, which votes along with 13 states on Super Tuesday on March 3. There's such an emphasis on winning California that the Sanders campaign internally refers to the "early 5" states. Early voting in California begins Feb. 3, the same day as the first-in-the-nation Iowa caucuses.

"We are the candidate who has a base," Sanders Iowa State Director Misty Rebik told the Des Moines Register, arguing that strong voter enthusiasm, like Sanders supporters have for their candidate, is critical to compete in Iowa's caucus system.

Still, Trippi said he gives Biden an "absolute edge" to go on to win the nomination over Sanders and other candidates because of his widespread support of black voters, a key constituency in any Democratic primary that will become more crucial as the race nationalizes.

"Like any of them, he's got to be able to break into Biden's strength with African Americans in order to get by him," he said.

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For much of the past year, buzz in the primary centered on the out-of-nowhere rise of Buttigieg, the emergence of Biden as an immediate front-runner and scrutiny of the latter's voting record. Some party insiders viewed Sanders as too far left to compete againand predicted his support from 2016 would shift elsewhere. Warren, D-Massachusetts, who also is trying to appeal to the progressive lane of the party, was widely seen as a more electable alternative to Sanders and enjoyed a steady ascension through the summer.

Warren, whose mantra is "big structural change," soared to a virtual tie with Biden in early October, around the same time Sanders suffered a heart attack. But days later, Sanders received a major boost when one of the most popular figures among liberals, U.S. Rep. Alexandria Ocasio-Cortez, D-New York, and fellow "Squad" members Rep. Ilhan Omar, D-Minn., and Rep. Rashida Tlaib, D-Michigan endorsed his candidacy. Their backing proved to be critical validation from the left — at a time when Sanders was trailing — to dispel speculation that Sanders' health might compromise his candidacy.

Ocasio-Cortez formally announced her support at a massive rally in Queens, New York, in front of some 25,000 people after informing the senator of her decision days before by phone while he was recovering in a hospital bed. She called it a "gut-check" moment for her.

"It would be dishonest of me not to let folks know how I feel about this race," Ocasio-Cortez said at the time.

Marking his return to the campaign trial, Sanders told the crowd, "I'm back."

As Sanders found his footing and Buttigieg picked up momentum as a center-left candidate, Warren began a consistent slide in polls that coincided with a heavily scrutinized rollout of her "Medicare for All" plan in November.

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"His big problem was Warren and her surge, and then she kind of took care of that," said Rachel Bitecofer, a political scientist whose model most accurately predicted the 2018 midterms. She called Warren's release of a health care plan with a $20.5 trillion price tag a "strategic blunder."

"Initially what I had expected to happen, as we moved through the fall and (Warren) was gaining momentum, is she would probably invert with (Sanders) and stay inverted," Bitecofer said. "Instead, what's happening so far is they're re-inverting and he's reasserting himself as the progressive frontrunner."

Bitecofer added: "Right now, if I had to say, he's going to be the advantaged progressive frontrunner heading into the actual formal primary."

Skeptics underestimated two strengths of Sanders — the loyalty of his supporters and his grassroots fundraising prowess. Polling consistently shows Sanders supporters are the least likely to jump to a different candidate. He's also set the bar in fundraising, posting an impressive $34.5 million haul in the fourth quarter, tops among all candidates, thanks to individual donations from 1.8 million people.

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But two hurdles remain for Sanders to secure nomination, according Bitecofer. Despite pulling ahead, Sanders still splits the progressive bloc of voters with Warren, which works to the advantage of Biden. Then there's his lagging numbers with African American voters, whose whose strong support for Biden has made him the frontrunner.

"If Biden can be in the top-tier in Iowa and New Hampshire — he doesn't have to necessarily win there, but just be close to the winners — and if he comes in and wins in South Carolina and has a great Super Tuesday, it puts him in a very good spot to win the nomination," Bitecofer said.

She called the hurdle Sanders faces a "mathematical fact" — if he can't turn around numbers with voters of color, he can't win the nomination.

A poll taken Dec. 30 through Jan. 5 from Morning Consult found Biden had support among 42 percent of black Democratic primary voters and Sanders in second among African American support, with 24 percent.

But Sanders performs better among young black voters. A poll from the BlackPAC political action committee, reported last month by Politico, found Sanders with support among 30 percent of black voters ages 18 to 24 and 25 percent among voters 25 to 39, besting Biden in both categories. The same survey had Biden leading Sanders among all black voters 38-15 percent.

Also notable for the Sanders campaign, the Morning Consult poll found Sanders leading among Hispanic voters, with 36 percent, up 7 points since last month, topping Biden's 24 percent among Hispanic voters. Support among Latino voters will be key in Nevada, California as well as Texas, another Super Tuesday prize where Biden is leading.

Jeff Weaver, a senior advisor for the Sanders campaign, said these trends show Sanders can win the nomination. In 2016, Clinton capitalized on support among voters of color and older voters to carry herself to victory after a drawn-out primary fight.

"The coalition that he's building is a multi-racial coalition that starts off much bigger than the 2016 coalition that he put together with much more potential to grow," Weaver said "In addition, we are continuing to reach out to working-class voters from every community to try to get folks involved in the political process who have given up on traditional politics."

Seemingly with an eye toward Biden's frontrunner status, Sanders has increasingly gone on the attack against Biden, slamming the former vice president's record as "so weak," singling out his support of NAFTA, past statements about Social Security and vote for the Iraq War.

In addition to overcoming Biden, Sanders will have to unite all factions of the Democratic Party.

The Associated Press reported this week about growing alarm raised by party insiders about a possible Sanders candidacy. That includes former aides of President Barack Obama who have warned that his push for "Medicare for All" and other progressive proposals won't play well in battleground states. Several former Obama aides have put their support behind Buttigieg.

“You need a candidate with a message that can help us win swing voters in battleground states,” Rahm Emanuel, former mayor of Chicago and Obama chief of staff, told the AP. “The degree of difficulty dramatically increases under a Bernie Sanders candidacy. It just gets a lot harder.”

Weaver rejected the suggestion that Sanders is not electable in a general election against Trump.

"A lot of these establishment Democratic consultants and lobbyists in Washington, these are the people who lost to Donald Trump in 2016," Weaver said. "I think that they are poorly placed in terms of being able to analyze the current race."

He said the Washington establishment, both Democratic and Republican, know that a Sanders presidency will "erase much of their relevancy in the American political process," adding that, "For many of those people, Bernie Sanders is a much more terrifying figure than Donald Trump."

"What we have got to do is nominate somebody who speaks authentically to the needs of working class people in every zip code and put that person up against Donald Trump who made promises about how he was going to protect the working class but has betrayed the working class in this country," Weaver said. "That's the way Democrats will win the White House."

Staff writer Nick Coltrain contributed to this report.

Reach Joey Garrison and on Twitter @joeygarrison.