Editor’s Note: Julian Zelizer, a CNN political analyst, is a professor of history and public affairs at Princeton University and author of the forthcoming book, “Burning Down the House: Newt Gingrich, the Fall of a Speaker, and the Rise of the New Republican Party.” Follow him on Twitter: @julianzelizer. The views expressed in this commentary are his own. View more opinion at CNN.

CNN  — 

As promised, Senator Bernie Sanders is shocking the political establishment. The candidate who many thought had no chance of winning is polling extremely well in Iowa and New Hampshire and several other states. According to a new CNN/UNH poll, he has a nine-point lead in New Hampshire.

Julian Zelizer

Former Vice President Joe Biden, the presumed frontrunner, is waking up to the reality that he is running neck-and-neck with the senator from Vermont. The other top candidates, such as Senator Elizabeth Warren and former Mayor Pete Buttigieg, are struggling to keep up with him. They are likely experiencing the same sinking feeling many Republicans felt in 2016, when they awoke to the reality that Donald Trump was stronger than the party establishment seeking to contain him.

The Sanders juggernaut does not appear to be a flash-in-the-pan candidacy. It is true that Sanders has had trouble expanding his base of support, but for several months now he has remained in the top tier in the polls, along with Biden, Warren and Buttigieg. It has become increasingly possible to imagine his winning the nomination.

How did a candidate who is so risky–as his supporter Alexandria Ocasio-Cortez admitted during a rally–work his way to the very top?

The most important element of Sanders’ surge is that his candidacy is built on top of a movement. Sanders subscribes to a vision of electoral politics which stipulates that campaigns need to be built from the bottom up.

In an age of candidate-centered elections, what was apparent in 2016 remains clear today: The senator has amassed a loyal and devoted cadre of followers who imagine that they are trying to transform the fundamentals of politics, not just win an election. Some of his main surrogates, such as AOC, are products of the movement he built in 2016.

The passion that Sanders commands from his supporters is intense and it has created a solid foundation for his candidacy that prevents the dramatic swings others have experienced.

The movement serves as a political weapon in the caucuses and primaries. Sanders’ supporters and surrogates are known for their quick and fierce response to his critics. Whereas this kind of support has elicited considerable frustration among his opponents, it is has been an asset in the heat of the campaign that supplements his own work at rallies and in the media. When candidates take on Sanders they have to take on everyone around him.

Sanders offers the kind of moral clarity, even if simplistic, which can be extremely appealing in an era where nothing seems trustworthy. At a moment when the President of the United States spews out disinformation on a daily basis and many Democrats seem to go back and forth on key questions–depending on which way the wind blows–there is something powerful about a candidate who has been saying the same thing, fighting the same fight, since early in his career. Though he clearly stands to the left of his party’s mainstream, many of his core ideas are connected to core liberal traditions, including national health care and progressive taxation.

Trump has also upended American politics in a way that gives someone like Sanders a fighting chance. The triumph of this kind of Republican leader and his ability to get away with what he does in the White House has poured cold water on the “reasonable” campaign experts who praise the promise of compromise and centrism. As the status quo seems so totally broken with President Trump running around in the Oval Office, there are more Democrats who are willing to “not play it safe,” as AOC urged her Iowa audience.

Sanders also benefits from the thinness of Biden’s support which has largely hinged on the elusive trait of “electability.” From the start, most experts have agreed that Biden’s strongest attribute has been the perception among voters that he is best positioned to defeat Trump.

Even with his gaffes and missteps, this view from the electorate has undergirded his strong numbers in most of the states. It has led many Democrats to rally around him even if they are not particularly excited about his vision. But that source of strength also makes Biden vulnerable and has created a dynamic field. When serious concerns emerge about his electability, it has created opportunities for the surges that we have seen.

With all of his liabilities, Sanders brings to the table his own arguments of electability. The passionate movement that he can count on offers a weapon to be used against Trump’s fierce campaign tactics. The senator’s ability to appeal to working class white Americans, as well as younger Latinos and African Americans, suggests that he could potentially build the kind of coalition that Democrats will need for victory. The attributes – such as his aggressive support base and resistance to compromise – that turn off so many Democrats, like Hillary Clinton who has remarked that “Nobody likes him,” could be seen as important in a campaign against President Trump.

Of course, early success will produce an even fiercer counter-mobilization that Sanders will have to survive. The vetting about his record will come fast and furious if he starts to tally up wins in key primary states.

Candidates such as Mike Bloomberg or Amy Klobuchar could easily experience a surge of support if centrists in the party start to fear that Sanders, and his policies, are unviable. Senator Elizabeth Warren could emerge as the only candidate capable of uniting left and center. All of these developments could start to undercut his gains.

Is Sanders an enormous risk who might end up costing Democrats the White House? He might be.

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    But the fact that Sanders is doing well right now shouldn’t come as a total surprise. There are solid reasons behind his growing success and they go far beyond the dynamic that we have seen in the past of candidates becoming “hot” at a given moment. The Sanders surge is rooted in core elements of the way that the senator practices his bottom-up politics and realistic fears of what it will take to defeat this president in 2020.

    It grows out of a genuine discontent in the electorate about the broken state of our politics.