DAVENPORT, Iowa (AP) — Vermont Sen. Bernie Sanders is quieting critics who questioned whether he could recapture the energy of his upstart 2016 campaign, surpassing his rivals in early fundraising and establishing himself as an indisputable front-runner for the Democratic presidential nomination.

Less than two months into his second White House bid , no other declared candidate in the crowded Democratic field currently has amassed so many advantages: a $28 million war chest, a loyal and enthusiastic voter base and a set of clearly defined policy objectives.

That puts Sanders on markedly different footing than during his first White House run, creating new challenges for a candidate whose supporters relish his role as an underdog and an outsider. He now carries the weight of high expectations and will face heightened scrutiny over everything from the cost and feasibility of his government-funded policy proposals to his tax returns, which he has not yet released. He initially blamed "mechanical issues" for the delay, and his campaign now says he wants to wait until after the April 15 tax filing deadline to fulfill his promise to release a decade worth of returns .

Sanders has largely embraced his new front-runner status. More than any other candidate, he draws explicit comparisons with President Donald Trump in his campaign remarks, previewing his approach to a general election faceoff with the incumbent Republican. Behind the scenes, Sanders is also building out a larger, more diverse campaign operation, responding to criticism that his 2016 organization skewed too heavily white and male. Campaign officials say the 2020 campaign staff — roughly 100 people and growing — is majority female and 40 percent people of color.

Still, Sanders' message and style hasn't changed from 2016, when he stunned many Democrats by mounting a formidable challenge to Hillary Clinton and besting her in more than 20 primary contests.

After briefly acquiescing to his advisers' suggestions that he reveal more about his upbringing and personal history, Sanders has returned to his comfort zone: delivering lengthy campaign speeches chockablock with the same policy prescriptions he campaigned on during the 2016 campaign. In Davenport, Iowa, on Friday night, Sanders spent 63 minutes outlining his views on health care, criminal justice reform and economic inequality.

"With your help, we are going to complete what we started here," Sanders told the 1,200-person crowd, referring to his virtual tie with Clinton in the 2016 Iowa caucuses.

Sanders' approach underscores his belief that his success in 2016 was not a fluke or simply a function of being the next best alternative to Clinton. His advisers argue the populist economic message Sanders has espoused for years, often in obscurity, has now been embraced not only by a slew of his Democratic rivals, but also Trump.

"Donald Trump campaigned on economic terms as faux Bernie Sanders. It was taking his language and selling it to the American people," said Faiz Shakir, Sanders' campaign manager. "And now how do you defeat faux Bernie Sanders? You defeat him with real Bernie Sanders."

Sanders owes some of his fast start to the fact that he never really stopped running for president after the 2016 campaign. Our Revolution, the political group Sanders launched after the campaign, has collected information on voters and held events in early voting states since the last election. Sanders was also active in the 2018 midterms, throwing his support behind progressive Democratic candidates across the country, though many were defeated.

"He spent 2018 lifting up progressives all over the country," said Rebecca Katz, a progressive Democratic consultant. "Even though many of them did not win, it was appreciated, it was movement building and it was a different calculation than most politicians make."

Despite his strong launch, Sanders' current standing atop the Democratic field is not entirely enviable. Presidential primaries are long and turbulent, and past elections underscore how many early front-runners have been tossed aside before the first votes are cast. Former Vice President Joe Biden has signaled his expected presidential campaign would serve as a centrist check on Sanders' brand of progressive politics.

And though Sanders' $18 million first-quarter fundraising haul far outpaced the rest of the Democratic field, some rival campaigns breathed a sigh of relief, having anticipated the Vermont senator would clear $20 million or more.

"He did very well. He could have done better," said Mo Elleithee, who advised Hillary Clinton's 2008 presidential campaign and now runs the Georgetown University Institute of Politics and Public Service.

Sanders also still has to prove that he can overcome some of the same vulnerabilities that contributed to his defeat in 2016.

Chief among them will be bolstering his standing with black voters, one of the most important constituencies in Democratic politics. Black voters overwhelmingly sided with Clinton in 2016, halting Sanders' momentum when the contest moved into more diverse states. He lost the South Carolina primary by a staggering 48 points.

Some of Sanders' top advisers dismiss the notion that he'll face similar problems in 2020, noting that he has spent time building relationships with black leaders in South Carolina and other Southern states. He's also sharpened his campaign message on criminal justice issues and racial inequality.

"I understand that a lot of people took a lot of things out of the South Carolina results," Shakir said. "We are going to continue to court and address these issues directly, but we are operating with a great deal of confidence that this is going to be a particular demographic that supports Bernie Sanders at the end of the day."

Perhaps Sanders' biggest challenge is overcoming skepticism among voters who may be partial to his focus on economic inequality but fear that nominating a 77-year-old self-described democratic socialist would put Democrats in a weak position against Trump in the general election.

"That's a thing that scares me about him," said Gwen Hobson, a 70-year-old Democratic voter, who attended Sanders' rally on Friday in Davenport.

Yet some of Sanders' longtime supporters say their enthusiasm for him is unshakable. In Davenport on Friday, several voters donned faded t-shirts from Sanders' 2016 campaign. Melita Tunnicliff, 57, wore a button she bought during that campaign with Sanders' photo and the phrase "Not For Sale."

Asked if she was open to other Democratic candidates this time around, Tunnicliff shook her head no.

"I've been waiting for Bernie," she said.

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