The senator has effectively not stopped campaigning since 2015 as the political landscape has shifted – but can he grow his 2020 support?

There is a saying in US politics, perhaps truer now than ever: every senator looks in the mirror and sees a president looking back. Before Bernie Sanders ran for the White House, his rumpled appearance and shock of unruly white hair suggested that this US senator gave his reflection no more than a fleeting glance each morning.

And yet, nearly two months into his second presidential run Sanders sits atop an unsettled Democratic field. It is a remarkable transformation for a democratic socialist from Vermont, deep into his 70s, who spent most of his decades-long career in the political wilderness.

Sanders has amassed the largest first-quarter fundraising haul, building a $28m war chest. His campaign already has about 100 staffers and continues to draw the kind of large, energized crowds that were the lifeblood of his insurgent 2016 campaign.

Polls of Iowa and New Hampshire Democrats placed him on top of those candidates who have formally declared, trailing only former vice-president Joe Biden, who is widely expected to enter the race soon. And this week, he reintroduced his signature Medicare for All healthcare bill with four Democratic 2020 rivals as co-sponsors.

He’s one of the leading candidates … because his platform is now where the center of energy is in the Democratic party Waleed Shahid

“Bernie being the frontrunner should come as a surprise to no one,” said Rebecca Katz, a progressive consultant from New York. “He has spent the last four years energizing his base.”

Sanders has effectively not stopped campaigning since he began in 2015. After Donald Trump’s election, he traveled to dozens of cities rallying opposition to Republican attempts to repeal the Affordable Care Act. During the midterms, he crisscrossed the country for progressive candidates.

Yet the political landscape has shifted considerably. There are 18 Democrats running for president, pitting him against younger and more diverse contenders.

“There is an embarrassment of riches in our field,” said Karine Jean-Pierre, chief public affairs officer for MoveOn.org who worked on Barack Obama’s campaigns in 2008 and 2012. “This time around he’s not the only person in the race talking about progressive issues.”

Being the frontrunner at such an early stage can be a blessing and a curse.

“Four years ago at this stage we were talking about Jeb Bush,” Jean-Pierre said, of the 2016 Republican primary. “And when Trump announced, he was polling at 1%. History tells us it’s way too early to tell.”

‘Like finding out Hamburglar is a vegan’

Sanders’ elevated status has attracted increased scrutiny of his policies and his past. He is under pressure to release his tax returns, which several of his competitors have done, at the same time as House Democrats are pursuing Trump’s taxes. Sanders has promised to release 10 years of returns by Monday, which is Tax Day.

The senator acknowledged earlier this week that the returns would show that he is a millionaire, owing to a “bestselling book”. Late-night talkshow host Jimmy Fallon said the revelation that the candidates who has railed against concentrated wealth for years is a millionaire was “like finding out the Hamburglar is a vegan”.

Facebook Twitter Pinterest Bernie Sanders introduces a new version of his Medicare for All plan on Capitol Hill in Washington DC with Kirsten Gillibrand and Ed Markey on 10 April. Photograph: REX/Shutterstock

Another new dynamic: Sanders’ competitors sound a lot more like him than was the case four years ago. A number of contenders embrace his economic policies and echo his populist rhetoric. But Waleed Shahid, communications director for Justice Democrats, a group that helps elect progressives, doubted Sanders would have trouble distinguishing himself.

“He’s one of the leading candidates in part because his platform is now where the center of energy is in the Democratic party,” Shahid said. “His popularity is perhaps less about Bernie himself and more about the long track record and authenticity he brings to his policy platform alongside a populist approach that names clear enemies in America’s oligarchy.”

Donald Trump wins if voters think Democrats have gone too far left Jim Kessler

Some worry Sanders’ populism will put the party at a disadvantage against Trump.

“Donald Trump wins if voters think Democrats have gone too far left,” said Jim Kessler, a co-founder of the center-left thinktank Third Way. Pointing to research conducted by the group, Kessler said candidates who are “ambitious but also pragmatic” are best positioned to assemble a big-tent coalition needed to extinguish Trump’s right-wing populist pitch.

This weekend Sanders is traveling through midwestern states that voted Trump in 2016. He will argue that the path to the White House runs through the rust belt and say he is positioned to wrest these states back in 2020.

At a rally in Madison, Wisconsin on Friday night, Sanders accused Trump of being a “pathological liar” who has repeatedly failed to keep his promises.

“The biggest lie of all was when he said that he would stand with the working class of our country that he was on their side and that he would take on powerful special interests to protect working families,” he said. “What a monstrous lie that was.”

‘Can he grow his support?’

Sanders has moved to address the criticism that hurt his first run for president. Having concluded that his 2016 team was “too white and too male”, he has built a campaign that is majority female and 40% people of color, according to campaign officials. He has also implemented strict new sexual harassment policies in response to what former female staffers described as a toxic and misogynistic workplace.

Seeking to bolster his foreign policy credentials, Sanders led a bipartisan effort to end US military involvement in Saudi Arabia’s war in Yemen. He has also expanded his platform to explicitly address issues of racial inequality, after being accused of prioritizing economic issues.

He must contend with a weak showing with black voters in 2016. South Carolina, where black voters make up a majority of the primary electorate and where Sanders suffered a crushing 47-point loss, will be a pivotal test.

His campaign and allies argue that his deficit with black voters was more a function of low profile at the time, and that he has since built relationships with African American leaders. But he continues to draw largely white crowds.

“Sanders has to prove he can gain supporters,” said Jennifer Palmieri, who was communications director for Clinton. “He has the most support now of the announced candidates – the question for him is: can he grow his support?”

‘A really radical and crazy idea’

Sanders is no longer the scrappy underdog. Yet the message and the messenger have changed little. He is still the same finger-jabbing, policy-rattling democratic socialist from Vermont who launched a long-shot bid for president from a patch of grass outside the Senate, known as the swamp, before hurrying back inside to vote. That is part of his pitch.

Imagine the United States joining the rest of the industrialized world and guaranteeing healthcare to all people Bernie Sanders

“I have some good news,” Sanders told activists recently at the We the People summit in Washington. “We have come a long way over the last four years.” He then galloped through list of ideas he ran on in 2016 that he said were deemed “too radical” then and are now within the Democratic mainstream.

“You want to hear a really radical and crazy idea? This is so radical, I hesitate to even bring it forward,” Sanders said, his words thick with sarcasm. “Imagine the United States joining the rest of the industrialized world and guaranteeing healthcare to all people as a right not a privilege.”

For those who met Sanders when he was a politically-lonely socialist mayor in Ronald Reagan’s America, his rise has been remarkable to watch.

Facebook Twitter Pinterest Bernie Sanders waves at a campaign rally in Greenville, South Carolina, on 21 February 2016. Photograph: Erik S Lesser/EPA

“I remember in the 80s, Bernie and I would drive around to visit these small towns in Vermont,” said Jeff Weaver, a senior adviser who first worked for Sanders in 1986, when he ran unsuccessfully for governor as an independent. “We typically spoke to a room of about eight people – 25 people was a big turnout.”

Certainly not then nor at any point during the nearly two decades in which he worked for Sanders on Capitol Hill did Weaver envision what has come to pass.

The Vermont senator is “far ahead” of where the campaign was in 2016, Weaver said. He added that his early fundraising figures will allow Sanders to expand operations in the early voting states including California, and to compete in all of the Super Tuesday states, a luxury he did not have in 2016 and a “key advantage”.

“This race is a marathon, not a sprint,” Weaver said. “The candidate who will prevail is the one who can go the distance. Bernie was a long-distance runner in high school.”