Julian Zelizer is a professor of history and public affairs at Princeton University and co-author with Kevin Kruse of the new book "Fault Lines: A History of the United States Since 1974." The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Sen. Bernie Sanders launched his second presidential bid on Tuesday and turned heads by raising $5.9 million right out of the gate. Within 24 hours of his announcement, more than 223,000 people had donated to the campaign. The numbers easily outpaced his rivals. Sen. Kamala Harris, for instance, raised $1.5 million in 24 hours, while Sen. Amy Klobuchar received $1 million in donations in the first 48 hours of her campaign launch.

With numbers like these, Sanders has -- for the second time in four years -- taken Democrats by surprise. The Brooklyn native has been repeatedly dismissed as a far-out senior and socialist who represents the "Ben and Jerry" electorate of Vermont. "There's enough ageism, religious bigotry and reflexive horror at the idea of socialism among the broad electorate," wrote columnist Eric Zorn in the Chicago Tribune in 2016, "that, if he wins the nomination, Sanders would probably lose every state—even his home state of Vermont."

Despite the skeptics, Sanders has demonstrated yet again that he has the ability to draw the kind of passionate and loyal support that Democrats will need going into 2020.

Other Democrats who are in the campaign mix are moving quickly to quash his candidacy. Harris, one of the most promising challengers to emerge so far, rejected the label commonly associated with Sanders and told voters in New Hampshire earlier this week, "I am not a democratic socialist." Beto O'Rourke, who is still considering a campaign, told reporters that the country can't meet "fundamental challenges" without "harnessing the power of the market." These and other remarks have all been interpreted as a slam against Sanders

Although many Democratic candidates appear focused on beating Sanders, they should take a pause from the horse race to look at both his years in the Senate and the elements of his 2016 campaign that seem to give him such strong appeal. Given that President Donald Trump attracts resolute devotion from many of his supporters, Democrats should learn from some aspects of Sanders' example and use them to galvanize a strong base.

Here are key elements from Sanders' approach to politics that could benefit the other candidates.

Build a movement, not a campaign

Sanders understood from the start of his 2016 presidential campaign that his mission was to build a movement, not just run for office. He always framed his candidacy in terms of bringing together voters who were frustrated with the status quo and wanted to fight for a different vision of American society. In doing so, he tapped into the spirit of the 1960s activism that rocked American politics. Despite having what some critics say is a larger-than-life ego , Sanders offered a candidacy that made his followers feel as if they were fighting for something bigger than themselves.

The movement was essential to sustaining his campaign against Hillary Clinton through an improbable primary and caucus run. And it didn't stop after Sanders lost. He has continued to work with candidates running for Congress and has used his public platform to keep selling his message. Many of his key ideas have since been embraced throughout the Democratic Party, and the payoff was evident from a new crop of legislators elected in the 2018 midterms. New York Rep. Alexandria Ocasio-Cortez, for one, does not shy away from progressive visions.

Don't be afraid to present bold ideas

Say what you will about his socialism or his sometimes-abrasive personality, but Sanders is a politician who believes in putting forward big ideas. The senator is not cut from the Bill Clinton model of politics, where the goal is to aim toward the pragmatic center and position one's ideas in such a way that they are difficult to attack as leftist.

In contrast to a political orthodoxy in Washington which almost always pushes candidates in that centrist direction, Sanders simply refuses to twist himself into knots. He likes to give the electorate big policies to chew on. In 2016, he focused on Medicare-for-all , a higher minimum wage, free college tuition , campaign finance reform, and ending growing economic inequality . Although it was easy to mock the repetitious and unpolished style of his campaign rhetoric, it gave voters a menu of ideas that very much defined what his movement was about. Many voters were glad to hear from a candidate who aimed for the moon and believed that the goal of a political campaign was to offer inspiration and aspiration rather than a set of finely tuned positions that had been watered down after several rounds of focus-group tests.

Authenticity matters

Sanders, who ostensibly understood that he didn't look the role of a president with his disheveled professorial appearance, decided that he wasn't going to change much. He would give voters Bernie Sanders and hope that they liked what they saw. He didn't embellish his speeches with bombast and he walked onto the stage and into the television studio dressed as he would on any other day in his office.

There was a logic to what he did and it explains some of the constituents' passion for Bernie. Voters are sophisticated enough to see through staged events and scripted talking points, and they like to see a real person asking for their vote. On the campaign stage, Sanders acted exactly as he has done throughout his career. He is the grumpy and certain leftie who has a real point of view about how the world works and is insistent on telling everyone what he thinks. He knows what he likes and knows what he hates and refuses to stay quiet about those opinions.

From his entrance into presidential politics, he has insisted on being exactly who he is and letting the voters decide what they think rather than molding himself to appeal to their whims

Fight, fight, fight

Sanders doesn't back away from a tough fight. It took a certain amount of chutzpah for him to even run for president in 2016. It was crazy to think that he could take on Clinton or that he could actually put together a legitimate campaign. But he did.

And Sanders did not hesitate to play tough. He is a progressive who knows how to throw a political punch. Sanders mounted an incredibly aggressive campaign in 2016, ignoring the party leaders who asked him to stop his primary campaign to directly confront Clinton on some of her biggest vulnerabilities, from high-paying speeches to corporate donations to her campaign.

Since losing the Democratic nomination, Sanders has continued to fight back against President Trump. After Sanders threw his hat in the 2020 ring, Trump tweeted, "Crazy Bernie has just entered the race. I wish him well!" Sanders fired back and wrote , "What's crazy is that we have a president who is racist, a sexist, a xenophobe and a fraud." That is the kind of blunt talk Democrats will need to employ in the ugly general election campaign that is sure to come.

It's also important to note that Sanders has on both occasions refused to run for President as a third-party candidate. In doing so, Sanders has demonstrated that he knows what he is fighting for and can see the clear differences between Republican and Democratic policies. Though he certainly made things more difficult for Clinton in 2016, he conducted his fight in the legitimate arena of the primaries rather than risking a Republican victory in the general election by peeling away left-leaning Democratic votes.

While Sanders is certainly a formidable candidate, there are plenty of reasons why many Democrats don't like him. He is not registered as a Democrat. He leans too far to the left for some in the party. He is often vague on the details. His ego seems way too large. He lost in the 2016 primaries so there is no reason to believe he can do better this time. Female staffers have spoken out about allegations of sexual harassment, disparities in pay, and an uncomfortable professional environment while working for his 2016 campaign. His candidacy also seems like a throwback to the past at a time when a record number of women are running for the presidential nomination; his entrance in the race this time around will also likely steal away support from Sen. Elizabeth Warren in particular, who could be a much stronger general election candidate against Trump.

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All of these criticisms are valid and present issues to be vetted.

But Democrats need to stop living in the past and start using a different playbook to take on the future. Sanders' approach, disconnected from the candidate himself, offers 2020 Democratic hopefuls some useful lessons about how they can put together a campaign that energizes, inspires and mobilizes voters to give their party the best chance of winning the White House.