A total of 69 sandstone steps lead up to the Philadelphia Museum of Art, and ever since a certain Rocky Balboa bounded up them four decades ago while training for a boxing match, joggers and tourists come to emulate his feat.

In his own way, Bernie Sanders also wants to match Rocky and land a knockout punch. Pitched against a better-established, better-known rival, he was himself fighting in Philadelphia on Monday, battling for the very survival of his “political revolution”.

“I like Bernie. For my generation, he is exciting and he gets people fired up,” said Sean Carnahan, a medical student who had just run up the museum’s steps on a warm afternoon.

Tourists still run up the steps where 'Rocky' was filmed (MGM)

“But I think Hillary Clinton has more of a record of getting things done. I think Bernie has broad ideas, but he doesn’t have a record of achievement.”

And therein may lie the rub. Since he announced his insurgent, against-the-odds campaign campaign last summer, the Vermont senator has gathered large crowds, raised millions of dollars in small donations and energised younger voters. He has also forced Ms Clinton to rethink not only her campaign strategy but also a number of her policy positions, such as her move towards supporting a $15 per hour minimum wage.

Addressing a roaring crowd in a sports hall at Drexel University in Philadelphia on Monday evening, urged supporters to make sure they cast their vote for change. "This campaign is about standing up. It's about saying the status quo is not working," he said.

Speaking to a crowd of more than 1,000 people at the University of Pittsburgh earlier in the day, he said that young and poorer people needed to vote in higher numbers if anything was to change.

"That means every person here has got to understand that you are very, very powerful people if you choose to exercise that right," he said.

Tourists still flock to where the scene in 'Rocky' was filmed (Andrew Buncombe ) (Andrew Buncombe)

"Politicians don't listen to people if they are not involved. They listen to people who contribute hundreds of millions of dollars."

Yet for all of that drama, Mr Sanders is still trailing Mr Clinton as they each seek to secure the 2,383 delegates required to cement the Democratic nomination. She currently stands on 1,428 to Mr Sanders’ 1,153, and she has a lead of 516-39 in so-called super-delegates, officials who are not bound to vote for a particular candidate.

On Tuesday, five more states will vote, with a total of 384 delegates divided proportionally between the two candidates. The biggest prize is Pennsylvania, where Mr Sanders had two campaign rallies on Monday, and which has 189 delegates. Earlier he held events in some of the hardscrabble Pennsylvania towns that his campaign believe could be open to his message of reversing economic inequality.

The situation could barely be more serious for the 74-year-old Democratic socialist. As Mr Sanders was due to speak in a sports hall at Philadelphia’s Drexel University, polls showed Ms Clinton to be ahead of him in each of the five states voting on Tuesday. It is the city of Philadelphia where the Democrats will hold their convention this summer and formally elect their nominee.

According to an average of polls collated by Real Clear Politics, she leads in him in Delaware by 21 points, in Maryland by 24 points, in Rhode Island by around 3 points, in Pennsylvania by 17 points and has an advantage in Connecticut of 6 points.

Mr Sanders held a rally in Philadelphia ahead of five primaries on Tuesday (Andrew Buncombe ) (Andrew Buncombe)

For Mr Sanders to catch Ms Clinton, the Vermont senator would have to achieve the kind of landslide victories not seen since his victory over her in New Hampshire, back in February. There is already considerable talk in some quarters that the time is fast approaching - if it has not already been reached - where he ought to stand down and allow Ms Clinton and the party to concentrate on the general election and countering Donald Trump.

“His only realistic goal for a long time has been to accumulate delegates to maximise his influence with the eventual nominee,” said Larry Sabato, Professor of Politics at the University of Virginia.

“As Sanders says, he’s willing to help reunify the party if Clinton is ready to reach out and make some concessions.”

Over the weekend, Mr Sanders insisted that he still had a “narrow path” to victory, and said he was not planning to end his campaign.

“While we have a narrow path to victory, were going to fight for and through that path. We hope to win,” he told NBC.

“But I think the fact that we have shown that there is massive dissatisfaction in this country with the status quo, that people want to think bigger, that people understand that when you have 20 people in this country owning more wealth than the bottom 150m people, that people are showing in this campaign, the desire to stand up and fight back.”

And even with the dream of the political revolution he talked of remaining beyond reach, Mr Sanders’ supporters remain as true to him as he does to the policies he has supported since the early 1970s.