One of the lesser-known facts about John McDonnell is that he began training as a Catholic priest before deciding against the vocation in his teens when he discovered girlfriends and the disadvantages of celibacy. Does his party need some kind of miracle to win this election? It is not hard to find Labour people who think so.

“I do believe in miracles,” says the shadow chancellor. “But we don’t need a miracle.

“Last time, at this stage of the campaign, I was getting lots of phone calls from colleagues saying, ‘We’re going to be wiped out and [we’re] really worried’. We’re at that stage now. I’m getting the phone calls which are saying: ‘I think, at best, [Labour will be the] largest party [in parliament]’. Then people saying: ‘Actually, on the doorstep, it doesn’t feel like the polls’.”

He makes the Labour optimist’s case for the way in which the campaign is unfolding.

“Mobilisation on the ground is huge. Despite the weather. The drawback for us is that being a December election, evening canvassing is in the dark. So you’re approaching someone’s house with a torch and frightening each other on the doorstep. That makes it tougher. But the mobilisation is beyond our wildest dreams.”

Then there’s social media, where the Tories are making much more effort than they did in 2017.

“The Tories have invested so much more in terms of social media …they’re spending much more money than us … but keep cocking it up.”

A doctored video of Keir Starmer was followed by Conservative campaign headquarters rebadging its Twitter account as “factcheck UK” during last week’s leaders’ TV debate. “Student politics stuff,” sniffs McDonnell. Tory fakery, he argues, only delivers “more attention on our agenda – which is: you can’t trust the Tories”. On the doorstep, he claims, “it comes across the whole time. You can’t trust a word he [Boris Johnson] says. Boris is Marmite on the doorstep. Absolute Marmite”.

The trouble for Labour is that this is at least as true of Jeremy Corbyn, another leader who dramatically polarises opinion. There are people who love the thought of prime minister Corbyn. But, if the polls are anything like correct, there are more who are repelled by the idea. And their numbers have grown since the election in 2017.

The problems we face are huge. So it does require a huge response. We've got these weeks to get that message across' John McDonnell

McDonnell instantly blames elements of the media. “They’ve had that extra couple of years of a full brutal assault on him, all the time.” He has to believe that Corbyn-sceptical and Corbyn-loathing voters will reconsider the Labour leader as the campaign progresses. “Jeremy is doing his rallies all around the country …. You’ll see much more of Jeremy in the national media now. And not just being seen in a white coat, like Boris with the stunts with boxing gloves. Much more quiet conversations on the sofa … Just get across the image of the man. When they [the voters] see that, they’re reassured.”

Labour has just published its manifesto, a prospectus that doubles down on the 2017 version by adding a slew of huge additional spending promises. “It is radical,” says the shadow chancellor. More radical than the last one? “Yeah, in some ways it is.”

Last time around, the manifestos proved to be the inflection point of the campaign. Labour was energised by its launch; the Tory campaign imploded soon after theirs. McDonnell and the rest of the Labour leadership team are hoping that history will repeat itself.

“What happened last time, as soon as the manifesto was out there, that did start to turn things. Because we started getting on to real policies.”

In conversation with us, he announces another large spending commitment, one omitted from the manifesto, saying he will compensate older females, often called “the Waspi women”, who lost out when the state pension age for men and women was equalised. “We’re recognising this is a historic injustice that we think we have a debt of honour to address.” He puts the cost to the taxpayer at £58bn.

That’s on top of many other spending commitments that Paul Johnson, the director of the Institute for Fiscal Studies, calls “colossal” and incapable of being paid for simply by raising taxes on the rich and business. There’s also some polling evidence that the scale of Labour’s promises are leaving many voters unconvinced, because they don’t believe the party could possibly deliver them.

“I understand that,” he responds. “The problems we face, the issues that we face, are big. They’re huge. So it does require a huge response. We’ve got these weeks to get that message across. But you’re right. We talk about billions. People just switch off. So the issue for us is translating that into what does it mean in your local area … so people don’t think the big numbers are just unreal.”

Jeremy Corbyn in Stoke on Trent on Friday. McDonnell believes, by not forcing his own views on others, the Labour leader can bring people together over the vexed issue of Brexit. Photograph: Anthony Devlin/Getty

The Tories are hoping to win Labour seats in the party’s historic heartlands with Johnson’s cry of “get Brexit done”. Labour’s gamble is that “the salience of other issues is more important than Brexit”. McDonnell contends: “The two big ones, which is health and education, have a greater salience for the Labour Leave voters than Brexit itself …. It is about security as well – police cuts and stuff like that. Also, climate change does come up on the doorstep.”

Labour is now committed to a fresh referendum on Brexit that would put Remain up against whatever kind of new withdrawal deal a Labour government could negotiate with the EU. McDonnell tells us that the negotiations would be led by himself, Corbyn and Starmer. Crucial to any renegotiation would be the subject of immigration, which is known to divide senior figures in the party. Does he accept that to achieve the closer relationship with Europe that Labour says it would seek, Britain would have to accept a continuation of freedom of movement?

McDonnell turns opaque: “It will be a negotiating issue.” He goes on: “Of course, we welcome the principles of free movement, but we also recognise that there are issues that have to be addressed.” Whatever new deal a Labour government might come up with, he says he is a Remainer. “I’ve made this point time and time again. I can’t see a better deal than that [remaining within the EU], but the party will make the decision once we come to the deal.”

No one can say what Corbyn would do, because he is continuing to refuse to say whether he is for or against Brexit. On Friday night he said he would adopt a “neutral stance” in a referendum campaign. When Labour’s candidate for the premiership won’t express a view about such a defining question, doesn’t it make him ridiculous? McDonnell endeavours to make a virtue of it, saying that he thinks voters will warm to “the quiet calm of Jeremy trying to … bring people together. That’s where we’ve got to position Jeremy in terms of the rest of the campaign on Brexit. This is the consensus builder. This is the man who can bring us all together. By not forcing his own view down people’s throats, he’s the only one who could do that.”

In what some may take as an implied criticism of Corbyn’s performance in the TV debate, he adds: “We’ve got to make sure the message gets across more clearly.”

There’s less than three weeks to do it.