Boris Johnson and Jeremy Corbyn will meet tomorrow for the first of this election’s televised setpieces, and the first of two head-to-head debates between the Prime Minister and the leader of the opposition.

As I wrote when ITV first announced the debate, I think these debates are win-win for Jeremy Corbyn, and lose-lose for Boris Johnson, but I wanted to get my thinking down on paper so, when I’m wrong, I can’t wriggle out of it or look back with hindsight.

Boris Johnson is winning anyway

Every available scrap of data points in the same direction: unless something changes, Boris Johnson is on course to be elected with a parliamentary majority. A lot of attention was lavished on John Curtice’s comment that the chances of Labour winning a parliamentary majority are as close to zero as possible. That has essentially been true since the 2014 Scottish independence referendum reconfigured politics north of the border. Labour now requires wins on the scale of 1945, 1966, 1997, 2001 and 2005 in England and Wales to govern alone. It has never looked capable of that under Ed Miliband or Jeremy Corbyn.

The more important point was what Curtice defined as the point at which the electoral system ought to produce a Johnson majority – a six-point lead. That matters because while we have a group of pollsters showing a big Tory lead and a group of pollsters showing a small Tory lead, both groups are showing the Conservatives at or ahead of what Curtice thinks the danger zone is.

Why does that matter? Because, ultimately, the debates are a moment when the election could shift on its axis. They were the media event cited as the “most influential” by a plurality of voters in 2015, with seven million people watching the BBC’s seven-way TV debate. Both Conservatives and Liberal Democrat strategists alike believe that without Nick Clegg’s strong performance in the three-way 2010 televised debates, David Cameron would have been able to successfully squeeze the Liberal Democrats down to 30 seats, and with it win a small majority.

I’m not saying that these debates will definitely change the result – I am saying that the risk that they will is sufficiently large enough that agreeing to them when the format is not 100 per cent in your favour and you are ahead in the polls is a reckless and foolish move.

Head-to-head debates are difficult for the incumbent as a matter of course

The political incentives here are pretty straightforward: in a head-to-head debate – particularly in the United Kingdom, where our political system means that only the incumbent Prime Minister is likely to have any record of governing to actually scrutinise – there is nowhere for the PM to hide in a contest that ought, if the leader of the opposition has half a brain, be skewed in their favour.

As so often, David Cameron provides you with a pretty good playbook on how to play this if you are a Conservative: in 2015, he agreed to a seven-way debate with the leaders of the Labour Party, the Liberal Democrats, the SNP, Ukip, Plaid Cymru and the Greens. The political incentives here were skewed in a completely different direction, and the sitting Conservative prime minister had the trump card of being able to turn to the camera at the end of a long and bitty debate and say “only a majority for me can prevent the next five years being more arguments like this one”.

That holds no matter how good or bad at televised debates the candidates in question are.

As it happens, Jeremy Corbyn is also better at them than Boris Johnson

Over the course of his political career, Jeremy Corbyn has appeared in more panel discussions than I’ve had hot dinners, and has appeared in televised debates with the following people: Yvette Cooper, Andy Burham, Liz Kendall, Amber Rudd, Caroline Lucas, Angus Robertson, Leanne Wood, Tim Farron and Owen Smith.

He has never lost – every post-debate poll, every collection of pundits…on any metric you care to name, the Labour leader has emerged from these clashes as the winner.

In his political career, Boris Johnson has appeared in televised debates with the following people: Ken Livingstone, Jeremy Hunt, Rory Stewart, Michael Gove, Sajid Javid and Brian Paddick. He has never won.

Polls declared Livingstone and Stewart the winners on the nights of their respective televised clashes. I thought that his one-on-one debate with Hunt was a draw, but mine was one of the more pro-Johnson verdicts: the general consensus among those watching, many of whom were committed Johnsonites, was that Hunt had the better debate.

Of course, in those instances, defeat in the debates didn’t stop Johnson winning the subsequent elections that followed. But my point here isn’t that “Johnson has agreed to do a one-on-one debate and will therefore lose”, but that by agreeing to a one-on-one debate he is his taking a risk that he does not need to take, in which the prize on offer is unclear and the downside risk is large.

Even if Jeremy Corbyn loses, the debate serves his interests better than Boris Johnson’s

Form is not destiny: you’d have expected a well-coached and in-form Wolves team to take all three points at Arsenal, but instead they drew. If 100 Boris Johnsons debated 100 Jeremy Corbyns, then I think it’s likely that you’d have 50 Corbyn victories, 25 Johnson triumphs and 25 inconclusive draws.

But there are two prizes on offer for Corbyn in this debate. The first of which he has to actively seize: defeat Johnson and begin turning around the polls. The second he may get simply by turning up: by symbolising that in much of the country, if you want shot of the Tories, your only available option is to pull the big red lever.

There’s also an indirect benefit: there is a clutch of Conservative voters who will vote Liberal Democrat but only if the result nationally is not considered to be up for grabs. If Corbyn ends either or both of the televised debates looking dead in the water, then it increases the chances that those voters will vote Liberal Democrat, which increases the chances of Conservative losses to the Lib Dems.

Remember that Corbyn doesn’t have to do that well in this election, or even to summon up something resembling enthusiasm in the country at large to enter Downing Street. He just needs to hold what has and for the Liberal Democrats and SNP between them to take ten seats off the Conservative Party – not a particularly tall order.

The dates are also really badly chosen from the government’s perspective

Small but important: the first set of postal ballots will arrive in people’s homes in the immediate aftermath of the first debate on ITV this Tuesday and the second one-on-one debate takes place less than a week before polling day.

Debate bounces may be relatively short-lived, but Johnson has made a rod for his own back by agreeing to debates where the afterglow of a Corbyn victory may hang around for just enough time to be decisive twice over.

It’s not clear what Boris Johnson can win by doing these

The big question that anyone suggesting that it is the right call to do a one-on-one debate has to come up with a plausible answer to is: what would happen if Boris Johnson hadn’t agreed to do it? I’m persuaded, both by my anecdotal impression while covering it and the polls and studies afterwards, that not attending any debates hurt Theresa May in 2017.

But there’s a big difference between “not doing any debates” and “not doing head-to-head debates”. As so often, David Cameron’s playbook is a pretty good starting point for Conservatives: in 2015 he agreed to a seven-way debate.

The other question is: what does Johnson actually stand to gain from this exercise? He’s already winning this election according to everything we can see. It’s an act of supreme folly to facilitate a moment that could change everything, and while he might get through these debates unscathed, he equally might not.