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.

Biden greets Sanders with an elbow bump before the start of a debate in Washington in March 2020. They went with an elbow bump instead of a handshake because of the coronavirus pandemic.

Biden greets Sanders with an elbow bump before the start of a debate in Washington in March 2020. They went with an elbow bump instead of a handshake because of the coronavirus pandemic.

Sanders speaks to reporters in Burlington, Vermont, a day after Super Tuesday II. Sanders said it "was not a good night for our campaign from a delegate point of view" but that he looked forward to staying in the race and taking on Joe Biden in an upcoming debate.

Sanders laughs during a primary-night rally in Manchester, New Hampshire, in February 2020. Sanders won the primary, just as he did in 2016.

In a tense and dramatic exchange moments after a Democratic debate, Warren accused Sanders of calling her a liar on national television. Sanders responded that it was Warren who called him a liar. Earlier in the debate, the two disagreed on whether Sanders told Warren, during a private dinner in 2018, that he didn't believe a woman could win the presidency.

Sanders campaigns at the University of New Hampshire in September 2019. A few days later, he took himself off the campaign trail after doctors treated a blockage in one of his arteries. Sanders suffered a heart attack, his campaign confirmed.

Sanders looks at his notes as he watches President Trump deliver the State of the Union address in February 2019. That month, Sanders announced that he would be running for president again.

Sanders looks at his notes as he watches President Trump deliver the State of the Union address in February 2019. That month, Sanders announced that he would be running for president again.

Sanders brings a giant printout of one of Donald Trump's tweets to a Senate debate in January 2017. In the tweet, Trump had promised not to cut Social Security, Medicare and Medicaid.

Sanders speaks at a rally in Santa Monica, California, in June 2016. He pledged to stay in the Democratic race even though Clinton secured the delegates she needed to become the presumptive nominee.

Sanders speaks at a rally in Santa Monica, California, in June 2016. He pledged to stay in the Democratic race even though Clinton secured the delegates she needed to become the presumptive nominee.

Sanders speaks at a campaign rally in Ann Arbor, Michigan, in March 2016. He won the state's primary the next day, an upset that delivered a sharp blow to Clinton's hopes of quickly securing the nomination.

Sanders and his wife, Jane, wave to the crowd during a primary night rally in Concord, New Hampshire, in February 2016. Sanders defeated Clinton in the New Hampshire primary with 60% of the vote, becoming the first Jewish candidate to win a presidential primary.

Comedian Larry David and Sanders appear together on "Saturday Night Live" in February 2016. David had played Sanders in a series of sketches throughout the campaign season.

Sanders sits with rapper and activist Killer Mike at the Busy Bee Cafe in Atlanta in November 2015. That evening, Killer Mike introduced Sanders at a campaign event in the city. "I'm talking about a revolutionary," the rapper told supporters. "In my heart of hearts, I truly believe that Sen. Bernie Sanders is the right man to lead this country."

Sanders embraces Remaz Abdelgader, a Muslim student, during an October 2015 event at George Mason University in Fairfax, Virginia. Asked what he would do about Islamophobia in the United States, Sanders said he was determined to fight racism and "build a nation in which we all stand together as one people."

Sanders embraces Remaz Abdelgader, a Muslim student, during an October 2015 event at George Mason University in Fairfax, Virginia. Asked what he would do about Islamophobia in the United States, Sanders said he was determined to fight racism and "build a nation in which we all stand together as one people."

Seconds after Sanders took the stage for a campaign rally in August 2015, a dozen protesters from Seattle's Black Lives Matter chapter jumped barricades and grabbed the microphone from the senator. Holding a banner that said "Smash Racism," two of the protesters -- Marissa Johnson, left, and Mara Jacqueline Willaford -- began to address the crowd.

In July 2015, two months after announcing he would be seeking the Democratic Party's nomination for President, Sanders spoke to nearly 10,000 supporters in Madison, Wisconsin. "Tonight we have made a little bit of history," he said. "You may know that some 25 candidates are running for President of the United States, but tonight we have more people at a meeting for a candidate for President of the United States than any other candidate has."

In March 2015, Sanders speaks in front of letters and petitions asking Congress to reject proposed cuts to Social Security and Medicare.

In March 2015, Sanders speaks in front of letters and petitions asking Congress to reject proposed cuts to Social Security and Medicare.

Sanders and US Rep. Jeff Miller, chairman of the House Committee on Veterans' Affairs, walk to a news conference on Capitol Hill in 2014. Sanders was chairman of the Senate Committee on Veterans' Affairs.

Sanders and US Rep. Jeff Miller, chairman of the House Committee on Veterans' Affairs, walk to a news conference on Capitol Hill in 2014. Sanders was chairman of the Senate Committee on Veterans' Affairs.

Sanders speaks to reporters in 2010 about the Obama administration's push to extend Bush-era tax cuts. Three days later, Sanders held a filibuster against the reinstatement of the tax cuts. His speech, which lasted more than eight hours, was published in book form in 2011. It is called "The Speech: A Historic Filibuster on Corporate Greed and the Decline of Our Middle Class."

Sanders speaks to reporters in 2010 about the Obama administration's push to extend Bush-era tax cuts. Three days later, Sanders held a filibuster against the reinstatement of the tax cuts. His speech, which lasted more than eight hours, was published in book form in 2011. It is called "The Speech: A Historic Filibuster on Corporate Greed and the Decline of Our Middle Class."

Sanders chats with Dr. John Matthew, director of The Health Center in Plainfield, Vermont, in May 2007. Sanders was in Plainfield to celebrate a new source of federal funding for The Health Center.

Sanders chats with Dr. John Matthew, director of The Health Center in Plainfield, Vermont, in May 2007. Sanders was in Plainfield to celebrate a new source of federal funding for The Health Center.

Sanders takes part in a swearing-in ceremony at the US Capitol in January 2007. He won his Senate seat with 65% of the vote.

Sanders takes part in a swearing-in ceremony at the US Capitol in January 2007. He won his Senate seat with 65% of the vote.

Sanders sits next to President Bill Clinton in 1993 before the Congressional Progressive Caucus held a meeting at the White House. Sanders co-founded the caucus in 1991 and served as its first chairman.

Sanders sits next to President Bill Clinton in 1993 before the Congressional Progressive Caucus held a meeting at the White House. Sanders co-founded the caucus in 1991 and served as its first chairman.

Sanders reads mail at his campaign office in Burlington in 1990. He was running for the US House of Representatives after an unsuccessful bid in 1988.

Sanders reads mail at his campaign office in Burlington in 1990. He was running for the US House of Representatives after an unsuccessful bid in 1988.

In 1987, Sanders and a group of Vermont musicians recorded a spoken-word folk album. "We Shall Overcome" was first released as a cassette that sold about 600 copies. When Sanders entered the US presidential race in 2015, the album surged in online sales. But at a CNN town hall, Sanders said, "It's the worst album ever recorded."

Sanders, right, tosses a baseball before a minor-league game in Vermont in 1984. US Sen. Patrick Leahy, center, was also on hand.

Sanders, right, tosses a baseball before a minor-league game in Vermont in 1984. US Sen. Patrick Leahy, center, was also on hand.

Sanders takes the oath of office to become the mayor of Burlington, Vermont, in 1981. He ran as an independent and won the race by 10 votes.

Sanders takes the oath of office to become the mayor of Burlington, Vermont, in 1981. He ran as an independent and won the race by 10 votes.

Sanders, right, leads a sit-in organized by the Congress of Racial Equality in 1962. The demonstration was staged to oppose housing segregation at the University of Chicago. It was Chicago's first civil rights sit-in.

Sanders, right, leads a sit-in organized by the Congress of Racial Equality in 1962. The demonstration was staged to oppose housing segregation at the University of Chicago. It was Chicago's first civil rights sit-in.

US Sen. Bernie Sanders speaks at a campaign rally in Chicago in March 2019. Sanders, an independent from Vermont, is the longest-serving independent in the history of Congress.

US Sen. Bernie Sanders speaks at a campaign rally in Chicago in March 2019. Sanders, an independent from Vermont, is the longest-serving independent in the history of Congress.

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.

Big crowds, big expectations

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."

The Reddit candidate

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."

Working on appeal to Democratic establishment

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."

Pointing to the polls

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."