The surging popularity of Sen. Bernie Sanders Bernie SandersJacobin editor: Primarying Schumer would force him to fight Trump's SCOTUS nominee Trump campaign plays up Biden's skills ahead of Cleveland debate: 'He's actually quite good' Young voters backing Biden by 2:1 margin: poll MORE has done little to alleviate the chief concern that Democrats have about his presidential bid: Namely, that he's simply unelectable on a national stage.

The Vermont Independent has quickly closed the gap on frontrunner Hillary Clinton Hillary Diane Rodham ClintonJoe Biden looks to expand election battleground into Trump country Biden leads Trump by 12 points among Catholic voters: poll The Hill's Campaign Report: Biden goes on offense MORE in national polls, while overtaking the former State secretary in the early voting states of Iowa and New Hampshire. Supporters say his rising momentum and populist message will carry him to the White House.

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But Sanders, a self-described democratic socialist, has spent a career operating largely from the left-most fringes of the Democratic Party with which he caucuses, stirring worry that he simply couldn't compete against a Republican perceived as a more establishment figure.

"No matter how well you think of Bernie — and all of us do — … when the politics of it all hits the road, I don't feel — and I feel most members don't feel — that he can be elected," said Rep. Alcee Hastings (D-Fla.).

The doubts have nothing to do with policy.

Indeed, Sanders' career-long advocacy for economic and social justice — a vision of wider safety nets, higher wages, universal healthcare and corporate policing — overlaps almost directly with the policy priorities of Rep. Nancy Pelosi (D-Calif.) and her liberal-leaning Democratic Caucus on Capitol Hill.

It's a convergence the Democrats have been quick to hail.

"I'm proud of what Bernie is saying out there, and it's a reflection of what we fight for here," Pelosi said last week.

And yet there remains a lingering sense among many Democrats that a Sanders' nomination would spell doom for the party in 2016 — a sentiment highlighted by the fact that not a single Democrat in either chamber has endorsed the No. 2 primary contender.

"Bernie Sanders is raising some issues that are important," Rep. Steny Hoyer (Md.), the Democratic whip and a Clinton supporter, told reporters last week. "But I don't think there's an expectation that's he's going to be president of the United States."

The dynamics surrounding Sanders' campaign present Democrats with an uncomfortable question: If the candidate trumpeting the party's agenda most loudly and clearly is unelectable, what does it say about the agenda, itself?

Hastings, another Clinton backer, said the answer lies in political expediency. He said he supports Sanders' economic agenda to a tee. But he also remembers too well the losing presidential campaigns of liberals George McGovern and Eugene McCarthy — both of whom he supported in the Civil Rights era — as well as the saga surrounding Ralph Nader, the consumer-rights advocate turned third-party candidate he blames for securing George W. Bush's victory in 2000.

"Some argue, and I do, that Ralph Nader cost us that election … and I don't have time for that. And I think that's what members are saying: That I don't have time for fringes, at this point. And that's where Bernie is, and it's regrettable," Hastings said.

"Mine and Bernie's philosophies regarding the disparity of economic well-being of America's citizens [are] in direct alignment with each other. I agree with him — [but] I support Hillary Clinton."

Rep. Bill Pascrell (D-N.J.) suggested Sanders' fierce advocacy has left little room for compromise, even with other Democrats.

"I tend to remind my progressive friends … that we've got to work together in order to accomplish something. And you don't do that by isolating the other side in your own party," said Pascrell, who's endorsed Clinton. "In other words, it's my way or the highway. Sometimes I have that feeling about what Bernie is all about."

Rep. Lloyd Doggett (D-Texas), who has not endorsed a primary candidate, said Sanders is doing "a great job" bringing the Democrats' policy agenda into the public eye. But he's concerned how the "socialist" label attached to the senator will play in a national election.

"The fact that he's been on that ticket raises some questions in other parts of the country: 'Can anyone who has ever had that label as an official candidate, as distinguished from an Independent, [win]?'" Doggett asked. "That's a question that many of us have had."

The doubts surrounding Sanders' electability are hardly universal. A growing number of lawmakers are pushing back as Sanders gains prominence in the race.

Rep. Alan Grayson (D-Fla.), a 2016 Senate candidate who has not endorsed a primary contender, characterized Sanders as one of Congress's most effective legislators, saying all claims that he's unelectable are "politically motivated attacks" designed to undermine his bid.

"Bernie has the appeal of being able to demonstrate to people that he can get good things done. He is not some kind of liberal stick figure; he's someone who has a record of actual accomplishment," Grayson said. "If Bernie's the nominee, then Bernie very likely will be the next president of the United States."

Rep. Raúl Grijalva (D-Ariz.) echoed that message, warning that Democrats, by doubting Sanders' viability as a candidate, risk undermining the party's agenda.

"The expectation that he will fade, I think, is not true because the agenda he's putting forward, instead of tamping down momentum, it's increasing momentum. … He's ignited the base in a way that we haven't been able to do for six years," said Grijalva, the head of the Progressive Caucus who has not yet endorsed in the primary. "So I would be very careful to marginalize the man. Because in a sense then you're marginalizing the message."

Asked why Sanders hasn't won any Democratic endorsements, Grijalva predicted that would soon change.

"It'll come," he said.

Sanders has a long history of defying the odds, and at no instance more dramatically than now.

The 74-year old, who launched his candidacy in May with little fanfare, has seen his star rise quickly over the summer. He's raised tens of millions of dollars, mostly in small donations; his speeches have attracted the largest crowds of any candidate of either party; and, aided by an email scandal dogging Clinton's campaign, he has skyrocketed in the polls even as Clinton has fallen.

The latest CBS News/New York Times poll found that Clinton still holds a resounding edge, but her popularity has fallen from 58 to 47 percent in the last month as Sanders' numbers have jumped from 17 to 27 percent.

Rep. Luis Gutierrez (D-Ill.) predicted Sanders' support would only grow as people become more familiar with his message. He said Sanders is doing "a great job" making the case for the Democrats policy platform — "and articulating it better than Hillary Clinton, too."

Gutierrez, who has officially endorsed Clinton, suggested he's ready to spread his support around.

"I said I liked Hillary Clinton, but you know what? I like Bernie Sanders, too," he said.