The TV debate is Corbyn’s chance to turn things round. Will it be enough to go from ‘boring and wet’ to electable?

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There has never been a head-to-head TV debate between the candidates to be prime minister before. So Tuesday’s encounter between Boris Johnson and Jeremy Corbyn – ITV, 8pm – will make history. The only other full-dress TV debates, in 2010, featured three leaders, including Nick Clegg of the Liberal Democrats, who was given equal time with Gordon Brown and David Cameron.

In 2015, there was one TV debate in which Cameron took part, an unmemorable affair before the election campaign really started, in which he was at the end of a row of seven party leaders. And in 2017, there was another seven-way debate that was memorable mostly for Theresa May not turning up and sending Amber Rudd instead.

So Tuesday is important. It is no use traditionalists muttering that the party leaders debate each other at televised Prime Minister’s Questions in the House of Commons every week. It is not the same – and besides, Corbyn and Johnson have faced each other only three times at PMQs since Johnson became prime minister.

Nor is it any use complaining that TV debates suck the life out of election campaigns. This one doesn’t have much life in it to start with.

No, the problem with TV debates is the endless coverage of what Alastair Campbell used to call “processology”: the arguments about format and whether Jo Swinson should have taken part; the focus on one-liners and soundbites; the marking of the exchanges as if they were a game; the instant polls declaring a “winner”.

In my view, all these drawbacks are outweighed by the democratic gain. TV debates attract huge audiences. They allow millions of voters the chance to see the candidates to lead their government up against each other, given time to set out their case and to challenge each other’s.

Of course, we can wish there was more substance. We would like an analysis of the long-term trade deal that Johnson hopes to negotiate with the EU in just five months next year. We want to know whether free broadband or university tuition is a good use of public money.

But those who want detail can find detail elsewhere. One of the questions about substance is whether a leader can communicate it clearly and briefly. And questions of character are also questions of substance. A TV debate may be an artificial forum, but it is a testing one.

So let us see how they do. I suspect that Corbyn will do better than expected. I realise that this is speculation about spin and perception – the very opposite of substance – but I think that it is partly because Corbyn comes across as more concerned about the substance, whereas Johnson can never shake off the image of the “clown”, as he was described in a focus group this week.

Which makes it surprising, to get deeper into the “processology”, that the Conservatives haven’t tried to lower expectations of their leader’s performance. On the contrary, they tend to express confidence privately that Corbyn will be “crushed” in the debate.

That certainly fits with the campaign strategy, which is to be relentlessly optimistic, upbeat and energetic. The overriding imperative for Johnson is to do things differently from the way Theresa May fought the last election. That means taking part in the TV debates in the first place – which is something that the conventional wisdom used to say the frontrunner should never do, until May refused to do it and suffered horribly as a result.

And it means Johnson doing more interviews and more campaign events with voters than May did – or than Johnson himself did when he ran his safe campaign for the Tory leadership. All these things are a risk, especially with Johnson’s tendency to use an interesting word when a dull one would do, but it would be a greater risk to try to hide the candidate away.

That means that Corbyn has his chance. Expectations of his performance start at rock bottom. The focus group that thought Johnson a clown described the Labour leader as “useless”, “boring”, “communist”, “wet”, “old”, “scary” and “dodgy”. Everyone has forgotten how he confounded his image last time.

In the last election campaign he managed to cast aside the tetchy self-righteousness for the twinkly demeanour of calm reason. He even managed to get through a half-hour interview with Andrew Neil without a scratch, and that is high praise indeed.

But Corbyn needs to do well in Tuesday’s debate. The polls suggest a wide range of possible outcomes in parliament, but every one of them projects a majority for the Conservatives. At the last election, Labour’s big spending promises turned out to be popular – although they enjoyed the advantage of no one thinking they would be put into practice.

This time Labour has decided that spending twice as much money must be twice as good – although the early evidence is that, while people approve of free broadband, they have doubts about renationalising part of BT.

Still, it will be fascinating to see how Johnson, a fiscally incontinent Conservative, deals with being outspent by a rival populist.

And, if Corbyn isn’t able to turn the election campaign round in just one TV debate, there will always be another – the BBC is also staging a head-to-head debate between prime minister and leader of the opposition on 6 December, six days before polling day.