In Syntagma Square, all the way from the grubby marble stairs opposite the Greek parliament down to the bottom of the plaza, people have gathered. Some are holding flags – the red, white and purple flags of Syriza, the once-radical leftist party whose leader they have come to hear.

Hip-hop thunders from giant speakers. The air is heavy and hot. An expectant crowd has been kept waiting for over an hour in temperatures turbocharged by massive spotlights. By the time Alexis Tsipras appears, many are sweating profusely. Still they roar their approval. The countdown has begun.

This is the embattled prime minister’s main pre-election address in what has been an unusually low-key campaign for the snap poll taking place in Greece on Sunday.

It’s showtime, and Tsipras doesn’t disappoint. He gives it his all. “They think they’ve got rid of Syriza, they think they’ve got rid of the left,” he bellows from a podium that is also encircled by gay supporters waving rainbow flags. “Well, the battle is only beginning. We can do it! We can pull off the greatest reversal in history.”

Four years on and the charismatic insurgent – formerly the hope of progressives and scourge of establishment parties Europe-wide – has a battle, as never before, on his hands.

Syriza is on the ropes. The drubbing it received in European parliamentary elections in May – emerging 9.5 points behind the centre-right opposition in its first contest at the ballot box since 2015 – is being seen as a precursor for a defeat likely to be comparable, if not bigger.

“It’s been unexpected,” conceded Aristides Baltas, a philosopher and former culture minister who helped write the leftists’ manifesto as a founding member of the Coalition of the Radical Left, or Syriza. “Everyone’s shocked. We are hoping the undecided will change their mind.”

Kyriakos Mitsotakis, leader of the opposition New Democracy party. Photograph: Miloš Bičanski/Getty Images

But if polls have been consistent about anything in Greece, it is Syriza’s tanking popularity. The party that rode a populist anti-austerity wave to storm to power at the height of the country’s financial crisis is now a shadow of its former self. Surveys show its rival, New Democracy, is between eight and 13 points ahead.

On Monday the keys to Maximos Mansion, the ornate neoclassical building housing the prime minister’s office and official residence, will almost certainly be handed over to Kyriakos Mitsotakis, the no-nonsense former banker who heads New Democracy. The Harvard-educated son of a past prime minister, whose family has been likened to the Kennedys – his sister Dora was foreign minister, his nephew Kostas is Athens’s mayor-elect – the 51-year-old could not be more different to his leftist opponent.

But even if Tsipras knows this is his last hurrah – payback time for the failure to take on Europe and deliver on the promise to eradicate austerity – he is not going down without a fight. The cliffhanger threat of euro exit may have caused him to U-turn, embracing the neoliberal reforms he had once so dramatically denounced, but he is still in the ring, throwing punches and lunging left hooks.

On Friday, in Syntagma Square, it was time to get down and dirty. Returning to the rhetoric of confrontation, Tsipras derided Mitsotakis and his ilk as “princes”, saying they belonged to a class that personified the old guard and had brought the debt-burdened nation to the edge of bankruptcy. If elected, the conservatives would roll back the labour and pension rights Syriza had struggled to reinstate, impose yet more harsh measures, privatise public utilities and bring an axe to the public health and education systems, he warned.

“They will treat you not as citizens but clients. They are trying to be something they aren’t, so that people will open the door,” he said, accusing New Democracy of hiding its real programme. “Will you entrust your dreams to people who stole from you? Don’t let them take your dreams.”

It is an irony of history that the great populist experiment began in Syntagma, reappearing in other EU capitals years later. It was here that Tsipras – and Yanis Varoufakis, his flamboyant finance minister at the time – exhorted Greeks to say a “big no” to ultimatums and “no” to blackmail when the excoriating terms of a third EU-IMF rescue package were put to the electorate in a referendum on 5 July 2015. And it was here that thousands came to dance and rejoice when an overwhelming 61% did indeed vote no.

But a week later, the firebrand leader had turned his big no into a big yes, signing up to a €100bn EU-funded bailout programme demanding some of the deepest cuts any government had been asked to apply since the scale of Athens’s insolvency first became apparent in late 2009. Appalled by the about-turn, Varoufakis resigned. His MeRA25 party is contesting Sunday’s elections.

Yanis Varoufakis at a rally with his wife, Danae Stratou. Photograph: Simela Pantzartzi/EPA

Explaining the compromise in parliament, Tsipras likened negotiations with Brussels to war. It was either concession or leaving the eurozone, a catastrophic prospect for a small country on the periphery of Europe surrounded by unpredictable neighbours.

But critics blamed Syriza’s shambolic handling of talks – which had previously been embodied by Varoufakis’s confrontational style – for terms that included agreeing to enact austerity until 2060 and achieving extraordinarily high fiscal surpluses of 3.5% until 2022.

“It was the first time in my life I had ever voted. It wasn’t a good experience,” mused Vangelis Flotsiotis, a sociology student at Panteion University in Athens, who had rushed to cast a “no” ballot. “Why vote for someone who lies?”

At 22, Flotsiotis is typical of the young voter Tsipras is now desperate to attract. Despite record-high levels of unemployment – at over 40% among young adults, the highest in the EU – Flotsiotis wants to remain in Greece and has no desire to join the hundreds of thousands who have sought jobs elsewhere.

Syriza MPs are the first to say politics these days is rarely about true feelings, even if it is emotion that prevails at the ballot box. After all, it was they who not only endorsed policies that ran counter to their conscience – in contrast to predecessors the leftist party suffered no defections when controversial measures were put to vote – but often did so with alacrity and zeal. For a band of leftist radicals, Greens, feminists, Trotkyists and Maoists, who never thought they would come to power, governance has been a rollercoaster ride. Most say they can understand the disappointment that has replaced the optimism engendered by Syriza’s original slogan “hope is coming”.

In the highly choreographed setting of mass rallies and campaign speeches, the whys and wherefores of how a populist revolt could have lost such steam was not addressed by Tsipras. Instead, on Friday, he underscored his intention to move to the centre, appealing to supporters who had once belonged to the socialist Pasok to come on board.

Among former Syriza ministers there is soul-searching. While many are dismayed that voters seem so bent on punishing Greece’s first leftist government – one they insist went out of its way to help the vulnerable and poor – questions are also being asked. “Yes, mistakes were made,” said Baltas, “but I think there are a lot of little things, little streams, that lead to this point.”

Privately, cadres say Syriza’s bungled handling of the deadly wildfire in the coastal resort of Mati last summer significantly helped accelerate the government’s fall from grace. When the inferno engulfed the town, 103 people died and scores were left badly burned. But there is also distaste over the way the leftists behaved while in power. Tsipras took to smoking cigars and in one embarrassing instance was spotted holidaying with his family on a shipowner’s yacht.

The whiff of opportunism – highlighted by the cynical decision to join forces with the far-right Independent Greeks party – and a perception of high-handedness spurred charges that, in power, Syriza was not much better than the old regime.

Taking in the view of Syntagma from his office, Nikos Filis, a former education minister who is widely seen as Syriza’s conscience, laments what he describes as “a style issue”.

“There are issues, certainly, about the way governance was conducted,” he sighs. “More humility would have been good. But we also forgot to look after the middle class who were so affected by taxes. We focused on the poor, but in the process overlooked the backbone of any society.”

But, at 44, Tsipras is still young, he says. “Whether we are in power, or not, we are here to stay,” he smiles.

• This article was amended on Sunday 7 July to correct an error made in the editing process relating the name of Yanis Varoufakis’s political party.