Thousands of Bernie Sanders's supporters descended on Washington Square Park in Manhattan on Wednesday evening, bringing even more colour and energy to the streets of New York.

A young guy wearing his own balding Bernie wig entertains the lines of people with an impersonation of Vermont senator and his barking Brooklyn accent. On the other side of the street, a father holds up a megaphone for his son to shout: "I'm 'Feeling the Bern!'" – the 74-year-old's unofficial campaign slogan.

An estimated 27,000 people, most in their teens and 20s, attended Wednesday evening’s rally – the third biggest rally of the senator’s campaign – to cheer on the Democratic presidential candidate before faces the New York primary on Tuesday.

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It's a round he must win by a big margin if he is to keep alive his hopes of catching up with front-runner Hillary Clinton and blocking her from winning the party's nomination.

One of New York's most famous parks and nearby streets were thronged with people, while many more watched the Wednesday evening rally from the apartments and buildings of New York University surrounding the park.

The creative posters, bright costumes and painted faces reflect the electricity that the oldest candidate in the US presidential race has generated among young people disillusioned with politics as usual and a “rigged” economy that has left them with diminishing opportunities to get ahead.

It's in contrast to the carefully stage-managed Clinton rally in Baltimore last Sunday where one five-year-old's "Go Hillary, No To Trump" sign encapsulated a more sober mood.

“He is the first candidate in our lifetime to really represent something that’s outside self-interested, self-driven, ego-maniac politics,” said Ally Fernandez (23), a student and waitress from Sleepy Hollow, New York, praising Sanders.

Mystical, magical creature

Fernandez was dressed as a “feminist mermaid”. To her, Sanders, unlike other politicians, is “a mystical, magical creature” whose momentum (he has won seven of the last eight states), popularity and ability to mobilise young people she struggles to understand.

“He is a mermaid, just like me,” she said, laughing.

She captured the carnival atmosphere of a "Bernie rally". Only a Donald Trump rally comes close for drama, but for different, divisive reasons.

The attraction of young people to the self-professed democratic socialist – the former mayor of Burlington, Vermont, and an independent socialist backbencher in the US Congress since 1991 – is obvious.

His ambitious campaign pledges to provide free universal healthcare, and free college education and reduced student debt funded by a “speculation tax” on Wall Street, appeal to them. There is also his proposal to introduce a “living wage” of $15 an hour, equal pay for women and paid family leave, along with his plan to overhaul a corrupt system of campaign finance that has created a “rigged” economy serving just the “1 per cent”.

Although he trails Clinton by 250 nominee-selecting delegates in the Democratic race to the magic number of 2,383 – a lead that looks difficult to close in the Democratic race that awards delegates proportionally on votes – Sanders has won the youth vote by a landslide in the state contests so far.

Of the 20 states for which voter breakdown by age are available in exit polls, Sanders won the millennial vote in all but two: Mississippi and Alabama, Clinton's two biggest victories. The New York-born senator was even the choice of voters aged between 17 and 29 in 15 states where Clinton won the overall vote, in most cases by between 60 and 80 per cent.

Sanders is drawing out the first-time voters. In New Hampshire, Texas and Wisconsin, the number of millennial voters outstripped the "65 years and over", a demographic guaranteed to turn out in big numbers.

For Sanders’s young supporters, Clinton is seen as a flip-flopper, untrustworthy and dishonest; Sanders, on the other hand, is authentic.

Politically expedient

Olivier Plummer (18), from Queens, attending his first political rally, was holding a sign that took a swipe at what he saw as Clinton’s politically expedient support for the Black Lives Matter movement, the wave of activism grown out of protests against police brutality and racial profiling.

“She has shown throughout her history that she hasn’t voted on the side of African-Americans,” said the young black man.

“She has supported the war on drugs, privatisation of prisons. She has supported racist policies and policies against immigrants so I can’t trust her with her past history to follow through in support of African-Americans.”

Across town in a Times Square hotel, Clinton is the keynote speaker at Reverend Al Sharpton’s National Action Network but one young attendee, Susan Obatola (17), a high-school student at Benjamin Banneker Academy in Brooklyn, is not impressed with the former first lady.

“Even though black people are more in favour of Hillary, I feel that she is not focusing on the things that we are more concerned about,” said Obatola, pointing to the black community’s struggles with education and housing.

“She is just scraping the surface rather than going in depth and trying to understand where we are coming from.”

Sanders, in contrast, is seen by his young supporters as having lived it. His arrest as a 21-year-old University of Chicago student at a civil rights protest in 1963, and his participation in the March on Washington behind Martin Luther King jnr that same year, give him street cred with youngsters who have grown up with an Occupy movement that has taken to the streets to protest against the greed of Wall Street – one of Sanders's biggest targets.

"Bernie has been saying the same thing the whole time that he has been in politics. He is very consistent," said Samantha Scheid (22), a waitress who has travelled to the rally from neighbouring New Jersey.

“Hillary, I think, is a liar. Every time somebody says, ‘I like this’, all of sudden, she likes that too. She is a conformer, not a leader.”

Listening to young people

During his rally, Sanders acknowledges the reasons for the success of a campaign that caught the political establishment and pundits by surprise.

“This campaign is listening to young people,” he bellows, firing up the crowd from under the marble Washington Square Arch.

He points to the staple advice of parents and teachers to young people – “go out and get a good education” – which, rather than setting them up, leaves them nursing tens of thousands of dollars that takes them decades to repay.

“We should be rewarding people who get the education they need,” he said, drawing some of the biggest roars of the rally.

The senator dismissed Clinton’s persistent criticism that his policies are unrealistic and lack detail on how he proposes to execute such lofty plans.

“People have said, ‘Bernie you’re thinking too big. Your ideas are just, you know, just,” he said, hesitating and prompting the crowd to deliver his popular Brooklyn-ese catchphrase “Yuge!” They erupt in cheers.

Holding his own “Feel The Bern” sign “Bern”, Asher Gladstone, a precocious 14-year-old from Long Island, can’t vote for four more years but is fired up by Sanders’s burning ambition.

Clinton's stranglehold on the "super-delegates" – party leaders and elected officials – shows how the party establishment is "more connected to Hillary Clinton rather than Bernie Sanders, who is for the people", he said.

Attending his first political rally and standing next to his sister Sage (11) and brother Quinn (8), Gladstone predicts that the New York primary will be “very close”.

“If he does win, it will change the tide of the election,” he said.