The committed Democratic socialist and sometimes self-styled radical has consistently argued that his brand of populist politics can win elections. In Vermont, his rumpled persona and righteously indignant approach to economic inequality helped him climb from mayor of Burlington to congressman and U.S. senator.
Now he'll find out whether he can have the same success nationally.
The first Democratic debate of the 2016 presidential season on Tuesday — sponsored by CNN and Facebook — offers Sanders his best chance to argue that his campaign is not merely a summertime lark fueled by fickle millennials, but instead a sturdy and battle-ready movement that can span 50 states and capture the nomination and the White House.
Ahead of the showdown in Las Vegas, Sanders spent the weekend making the case he's not out of the mainstream, repeatedly arguing that it's "not a radical idea" to think someone should be paid a livable wage or that students should not go into a lifetime of debt. "I don't think this is a leftist, extremist position," he said at a Saturday rally in Boulder, Colorado, referring to a $15 minimum wage.
Hillary Clinton's failure to quiet the questions surrounding her use of a private e-mail account coupled with her establishment credentials have provided an opening for Sanders, who nearly matched her quarterly fundraising total with a $26 million haul.
"He has been waiting for this his entire career," said Greg Guma, who wrote "The People's Republic: Vermont and the Sanders Revolution." "This is Bernie getting his close-up."
Since announcing his candidacy in April, support for the 74-year-old Sanders has grown from 6% to 24% of registered Democratic voters, according to CNN/ORC polls. And he's surged to a lead in New Hampshire, the state that has provided crucial support to the Clintons in the past.
But that won't be enough to win nationally. Sanders trails badly against Clinton— and Vice President Joe Biden -- in South Carolina and Nevada, according to a CNN/ORC poll released Monday morning. His problem is particularly acute when it comes to minority voters. In South Carolina, only 4% of African-American voters back Sanders -- a number that jumps to 7% if Biden doesn't run.
So far, his response to any lingering doubts has been a simple one—massive crowds.
Over the course of roughly 30 days in the middle of the summer, 100,000 people turned out to see him, raising comparisons to Barack Obama's 2008 campaign. In Madison, Wisconsin, 10,000 people showed up to hear Sanders bash oligarchs and Wall Street fat-cats. In the run-up to the debate, 22,000 heard him in Arizona and Colorado this weekend.
"We were able to organize organically. We hadn't hired 50 advance people," said Tad Devine, a Sanders adviser, recalling the early crowds. "The crowds created this excitement and enthusiasm and gave us this incredible platform to begin to organize states. The thing that made it so interesting and attractive to us is that we were able to marry technology to it."
Even the candidate seems surprised by his appeal. Every time he takes the stage to see thousands of people waiting to see him, he does the same thing: Mouths the word "whoa" to himself.
But his primetime showdown with Clinton won't come in front of 20,000 people on a college campus. And in Clinton, he will face a seasoned debater and policy wonk, who many thought got the better of all of her opponents in 2008. This means that for Democratic -- or Republican -- voters unfamiliar with him, their first peek at Sanders won't come in the type of setting that he has used to build momentum this year.
"He is very relaxed and familiar, like a college professor just getting up there speaking," said John MacGovern, a Republican who debated Sanders the 2012 Senate race. "But he can be very aggressive, very strong and he speaks loud and in a rat, tat, tat, tat, drum beat."
Guma, who once played Sanders in debate prep for his race against Vermont Governor Madeleine Kunin in 1986, said the senator has softened his style a bit.
"Bernie is modulating his temperament. He is a brusque person who has a tendency to be dismissive, but he is accentuating the winning side of his personality," he said. "He is trying to convince people that he is a candidate with mass appeal."
Before becoming a presidential candidate, Sanders, who friends say has studied the media and how messages and narratives are created, carefully cultivated a young and progressive following, with appearances on Bill Maher's show where he sparred with Republicans. Weekly, for roughly a decade, he has joined popular progressive radio host Thom Hartmann for "Brunch With Bernie" town halls.
Sanders also steadily boosted his profile on social media — he now has almost 700,000 twitter followers and counts nearly 1.7 million likes on Facebook. And Sanders' Reddit footprint is unmatched—his dedicated subreddit has 117,000 users to Clinton's 760.
Indeed, his crowds often fit the Reddit demographic — young, male city dwellers.
"I would say a lot of my interest was spiked from Reddit, I subscribe to the Bernie Sanders subreddit so I've been just kind of following what is going on," said Chris Knapp, a software engineer from Tucson and one of the 13,000 people who heard Sanders Friday night. "It is a very active community. It is easy to just go to one source and keep up on all the information. It is a crowd sourcing of different news articles. That is how I found out about the rally tonight."
From June to August, Sanders nearly doubled his standing in the polls, grabbing nearly a third of the vote and much of the excitement. And Sanders' voters aren't lukewarm. In Sanders they see a kind of prophetic figure who may have once been the wild-haired and wayward stepson of the Democratic Party, but is now its most passionate spokesman.
As Sanders took the stage in Boulder, before a crowd of 9,000 on Saturday, a woman behind him held a simple sign: "Bernie: 30 Years of Truth."
Yet, for all of his talk of political revolution and stylistic and policy differences with the White House, Sanders sounds conventional and Obama-like when he talks about how he will govern and bend Republicans to his will.
"I think we can do it. And I think that's what the bully pulpit is about," he said Sunday on NBC's Meet the Press, suggesting that he would mobilize protestors to descend on Washington to support his agenda. "And that's what organizing effort's about. And that's what this campaign is about."
While the sheer size of his crowds has been his best electability argument, they have also made him a target. Confronted by Black Lives Matter activists this summer, Sanders initially showed his trademark stubbornness, refusing to engage as he stayed on his economic message to the exclusion of race. By comparison, Clinton gave a speech in December 2014 where she said "black lives matter," putting her ahead of her Democratic rivals, even as she had a tense exchange with activists later.
The interactions with young black millennials pushed Sanders to do something that he hadn't done in the past—create a campaign staff and message that mirrored the diversity of the Democratic Party. He added lines to his stump speeches about racial disparities in the criminal justice system and met with key stakeholders on the issue.
"I plead guilty—I should have been more sensitive at the beginning of this campaign to talk about this issue," he said in a New Yorker article this month, speaking about police brutality.
Sanders, once called a "homeless waif" by Bill Richardson while he was in the House, has been slow to gain establishment support, even as he makes the case for his electability. He just landed his first endorsement from a member of Congress -- Arizona Democrat Raul Grijalva -- while Clinton has well over 100 endorsements from Congress including over 60% of Sanders' colleagues in the Senate.
In August, he made his first ever appearance at a Democratic National Committee's quarterly meeting, where he argued that the party he once called "ideologically bankrupt" needs him.
"Democrats will not retain the White House, will not regain the Senate or the U.S. House, will not be successful in dozens of governors races all across this country, unless we generated excitement and momentum and produce a huge voter turnout," he said, clearly nodding to the size of his crowds. "That turnout, that enthusiasm, will not happen with politics as usual."
And the least poll-tested candidate in the field -- someone who to this day does not pay a pollster -- Is now touting polls.
"New polls confirm what we've known for a long time -- we can win in both the primary and general elections," he tweeted recently. Sanders' supporters have been invigorated, if not surprised, by his rise in national and state polling.
"I didn't know that there was this kind of thirst in this country for real political change," said Russell Mendell, a 30-year old from Boulder who stood with his friends and waited for Sanders in the baking sun. "I didn't know that people were paying as close attention as they are. ... Starting from where he started from to already be leading in some polls, it is phenomenal."
On banks and college affordability, he has staked out a position that is on the far left—he would break up the big banks and reinstate the Glass-Steagall Act and offer free tuition at public colleges.
But his record on guns has been mixed—he has opposed a federal law mandating a waiting period for gun purchases and voted against the Brady Handgun Violence Prevention Act in 1993. He has more recently said the goal should be to "move forward in sensible ways," and has started to highlight his more recent support for gun control legislation. On Meet the Press Sunday, Sanders said he's open to backing off his vote for a 2005 measure that shielded gun manufacturers from liability in lawsuits over shootings.
Ahead of the debate at two large rallies, Sanders has sharpened his message on immigration and guns, two issues that have the potential to trip him up against Clinton among progressives. Criticized by Latino elected officials for not mentioning immigration reform at all, Sanders now has a line in his speech about the need for comprehensive immigration reform, a push he helped scuttle in 2007.
Aides said that the debate will offer an opportunity to address the electability question, but a more formal speech might also be in the works.
"As we get more serious in this process, that is going to be an issue that a lot of voters raise and we've known that from the beginning," Devine said. "If you look at the history of the New Hampshire primaries and the Iowa caucuses, electability is an issue that those voters care a lot about—not only the ideas that candidates talk about but whether or not those ideas can be lead into action and Bernie will continue to talk about that."