Alexis Tsipras arrives late: he’s spent the morning sorting out wrangles over Syriza’s candidate list for this month’s election. As the polls currently predict he’s likely to win, this is no longer a side issue. If Syriza becomes the first far left government in modern Europe, those MPs will be asked to pull the trigger on a showdown with Europe that could change the economics of the whole eurozone.

Tsipras has pledged to end the austerity programme imposed by the troika – and at the same time negotiate the write-off of 50 per cent of Greek debt. So the obvious question is: what does he do if they say no?

“I’ve answered that thousands of times since 2012,” he says. “Syriza is the alarm clock that will wake the EU leaders out of their slumber. What we demand is a European conference, to tackle this European problem together, and there cannot be a solution without writing off a large part of the debt, a moratorium on repayments and a growth clause.”

That amounts to saying no repayments until there’s growth enough to revive the Greek economy, which from the look of the streets surrounding the Syriza party HQ, is obviously depressed.

Though the polls put Tsipras in the lead – on 33 per cent in an opinion poll tonight – the newspapers still treat him as a political outsider: “Tsipras dons the veil,” says one headline – referring to his liberal immigration policy.

But the biggest fear of the centrist parties who oppose him is that, if Tsipras confronts Europe, Greece will be forced out of the eurozone – especially if he cancels the austerity programme but still needs access to IMF, EU and private market loans.

He answers: “In reality we are not asking to borrow any new money. We have no intention of asking for new lending to repay old loans. Of course we’re going to negotiate with all of our partners so that we can confront together the common European problem of unsustainable Greek debt.

“And this is not the first time something like a debt write-off this has been implemented. It happened in 1953 in Germany. And I am wondering on what ethical grounds does Germany refuse a solution to the European problem, which it benefited from many years ago, when coming out of world war two, and when Germany itself had many open wounds?

“I am saying to the people of Europe, especially the people of northern Europe: we don’t want any more of your money. The money you have been giving all these years wasn’t spent to keep the Greek people on their feet. Instead it was used to recapitalise bankrupt banks, so that the banks and the financial system not only in Greece but the whole of Europe would not collapse. You gave Greece toxic money. Now it’s time to find a solution for the common good.”

This week the German newspaper Der Spiegel reported that the German government is no longer committed to keeping Greece in the euro – and is prepared to tolerate the so-called “Grexit” that scared everybody witless in 2012.

Tsipras laughs: “This is a discussion that died in 2012. It’s like reheated food. A dance of the zombies. The question is, are we going to religiously stick to the rules even if they are wrong and damage us, or the path that says ‘do whatever it takes to save our common home’, the EU, and more importantly our society, our people.”

Some in the financial markets believe Tsipras will buy time by getting the Greek banks to lend to the state – putting the onus on the ECB to try and stop it. I ask if he intends to use the banks to finance the state:

“If you mean treasury bills, then this is something all previous governments have done. And if deemed necessary it’s something a Syriza government might do as well.”

His opponents in the governing New Democracy party say he will use Greek savers’ bank deposits to fund the government. There’s been a furious row about this, and he denies it:

“If you’re talking about funding our deficit through the deposits of the Greek people you can forget about that. There is not a single chance of that happening in Greece. Syriza is coming to guarantee the deposits, not to threaten them. I want to make this clear, and warn those who insist on fearmongering – they will face the consequences of the law if they carry on.”

In the financial markets there is a growing acknowledgement that Syriza stands a chance of winning. The Greek stock exchange fell 2 per cent today. Financial experts acknowledge much of the 319bn euro Greeek debt is unpayable, and will have to be renegotiated whoever wins. But the EU plan was to get more pro-market reform in return – not a pro-welfare, pro-labour government of the left.

In German political circles people say privately: OK, we got massive debt relief in 1953, but in return we reformed our entire system and political culture.

I put it to Tsipras that there is no chance that he is the man to deliver this. He says, if the Germans were honest they would admit they backed the parties who got Greece into this mess:

“We are not responsible for the culture of corruption created by the governing parties. We want a state that stands by its citizens. We need to stop this carnival of tax evasion and avoidance. We’re not hiding anybody.”

If he wins, the next part of 2015 will be about Tsipras vs Merkel: as it happens, Frau Merkel is in London, visiting the British Museum, which contains the Elgin Marbles. I ask: which would he rather have – debt relief or the marbles back?

“Can I have both?” he laughs, adding: “We are going to demand debt reduction, and the money Germany owes us from world war two, including reparations, but we also want the marbles, which don’t belong to us but to everybody, and which need to come back to their home.”

If the current polls are right – and his 3 per cent lead holds through to election day – there is a strong chance Tsipras will write his own chapter on to the stones of Athens. However moderate and parliamentary Syriza has become, it remains – formally – a Marxist party with a central committee, a newspaper, and swarming through the cramped stairways of its small office, people who talk and look like activists, not ministers.

No democratic country in Europe has been governed by a party like that since world war two. Some of the Syriza members I spoke to are, privately, still not sure they even want to govern. But one thing is certain: when you look him in the eye, Alexis Tsipras most definitely does.

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