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.

General election: On the campaign trail Show all 114 1 /114 General election: On the campaign trail General election: On the campaign trail Boris Johnson drives a Union flag-themed JCB, with the words "Get Brexit Done" inside the digger bucket, through a fake wall emblazoned with the word "GRIDLOCK", during a general election campaign event at JCB construction company in Uttoxeter, Staffordshire AFP via Getty General election: On the campaign trail A sculpture of an owl is bedecked with an EU beret as Liberal Democrat leader Jo Swinson gestures during an interview at a general election campaign event in Bath AFP via Getty General election: On the campaign trail Boris Johnson (C) talks during a question and answer session on a general election campaign visit to Ferguson's Transport in the town of Washington, west of Sunderland AFP/Getty General election: On the campaign trail Boris Johnson tries to save a shot during a warm up before a girls football match while on the campaign trail in Cheadle Hulme, northwest England POOL/AFP via Getty General election: On the campaign trail Jeremy Corbyn makes cup of coffee as he campaigns in Barry, south Wales AFP General election: On the campaign trail Nicola Sturgeon poses as she launches the SNP campaign bus tour, at Queensferry Crossing in Edinburgh, Scotland AFP via Getty General election: On the campaign trail Boris Johnson reacts as he changes a wheel of a Formula One car during a Conservative Party general election campaign visit to Red Bull Racing in Milton Keynes POOL/AFP via Getty General election: On the campaign trail Britain's Prime Minister Boris Johnson visits a Christmas market in Salisbury REUTERS General election: On the campaign trail Liberal Democrat Leader Jo Swinson on the election campaign bus on route to Chelmsford PA General election: On the campaign trail Labour leader Jeremy Corbyn talking to a university worker outside Birkbeck/SOAS University of London, at the announcement of his parties of the plan for the extension of workers' rights PA General election: On the campaign trail Nicola Sturgeon plays a game with string during a visit to Perth and District YMCA Getty Images General election: On the campaign trail Brexit Party leader Nigel Farage picks up a copy of The Brexiteer during a visit to JPI Printers in Dinnington PA General election: On the campaign trail Boris Johnson, centre, talks to veteran James Gammer as he visits a veterans centre AP General election: On the campaign trail SNP supporters meet First Minister Nicola Sturgeon following her address during a St Andrews day election speech Getty General election: On the campaign trail Jeremy Corbyn has a photo taken outside Birkbeck/SOAS University PA General election: On the campaign trail Liberal Democrat leader Jo Swinson during a visit to the Cardiff United Synagogue PA General election: On the campaign trail Brexit Party leader Nigel Farage reacts as he attends an election debate ITV/Reuters General election: On the campaign trail Jeremy Corbyn wears a jacked displaying Labour's slogan 'For the many, not the few' EPA General election: On the campaign trail Prime Minister Boris Johnson on The Andrew Marr Show Getty General election: On the campaign trail Nicola Sturgeon plays with local children during a visit to the Jelly Tots & Cookies Play Cafe in Uddingston, South Lanarkshire PA General election: On the campaign trail Opposition Labour Party leader Jeremy Corbyn eats some chips in Whitby, northern England AFP via Getty General election: On the campaign trail Boris Johnson and Conservative MP George Eustice, left, tour 'Rodda's Clotted Cream' whilst campaigning in Cornwall Getty General election: On the campaign trail Jo Swinson during a visit to Dunkertons Cider Company, an organic and plastic free brewery in Cheltenham PA General election: On the campaign trail Jeremy Corbyn holds up unreacted documents with NHS staff. Mr Corbyn released uncensored official documents which he says provides confirmation that the NHS is on the table in trade talks with the USA EPA General election: On the campaign trail Nigel Farage meets locals in Barnsley market PA General election: On the campaign trail Nicola Sturgeon launches the SNP's general election manifesto Getty General election: On the campaign trail Labour Party leader Jeremy Corbyn addresses the audience during the launch of Labour's environmental policy in Southampton Getty General election: On the campaign trail Boris Johnson tries an apple as he visits Healey's Cornish Cyder Farm in Callestick AP General election: On the campaign trail Jo Swinson helps make a chicken tikka masala during a visit to the Ashoka restaurant in Bearsden, Glasgow PA General election: On the campaign trail Nigel Farage playing golf on a range at One Stop Golf in Hull PA General election: On the campaign trail Jeremy Corbyn addresses the audience during a climate emergency rally in Falmouth Getty General election: On the campaign trail Liberal Democrat leader Jo Swinson meets supporters during a visit to Dunkertons Cider Company PA General election: On the campaign trail Boris Johnson visits Chulmleigh College AP General election: On the campaign trail Labour Party leader Jeremy Corbyn as he appearing on the BBC political programme The Andrew Neil Show. The opposition leader defended his handling of anti-Semitism allegations after Britain's most senior rabbi suggested the issue was a "new poison" within the party that had been "sanctioned from the very top". In an unprecedented intervention ahead of next month's election, Chief Rabbi Ephraim Mirvis suggested Corbyn was "unfit for high office" due to his perceived failure to clamp down on the problem within the party BBC/AFP via Getty General election: On the campaign trail Liberal Democrat leader Jo Swinson alongside Julian Dunkerton, whose father founded and ran Dunkertons Organic Cider, during a visit to the company's organic and plastic free brewery in Cheltenham, Gloucestershire, whilst campaigning for the General Election. PA Photo. Picture date: Tuesday November 26, 2019. See PA story POLITICS Election. Photo credit should read: Aaron Chown/PA Wire Aaron Chown PA General election: On the campaign trail Prime Minister Boris Johnson launches the Conservative Party Scottish Manifesto in North Queensferry Getty General election: On the campaign trail Britain's Opposition leader Jeremy Corbyn poses with WASPI (Women Against State Pension Inequality) supporters after he outlined Labour's plans to repay the "historical debt of honour" owed to women born in the 1950's AFP via Getty General election: On the campaign trail Jo Swinson alongside the Lib Dem's candidate for Truro and Falmouth, Ruth Gripper, and founder of Finisterre, Tom Kay, during a tour of the British surfing brand, which creates sustainable surfing gear, in St Agnes, Cornwall PA General election: On the campaign trail Prime Minister Boris Johnson poses with sheep as he visits the Royal Welsh Showground, in Llanelwedd PA General election: On the campaign trail A coalition of NHS (National Health Service) campaigners led by "Keep Our NHS Public" and "We Own It", pose for photographs with masks of US President Donal Trump, right, and Prime Minister Boris Johnson carving up the NHS with a giant knife and fork, as they take part in a demonstration on Parliament Square. The campaigners are calling for the NHS to be taken "off the table" in a trade deal with the US by ending privatisation in the health service and restoring it to a wholly publicly owned and provided service AP General election: On the campaign trail Brexit Party leader Nigel Farage visits Plymouth Fisheries at Sutton Harbour Getty General election: On the campaign trail SNP leader Nicola Sturgeon gives her party's candidate for North East Fife Stephen Gethins a haircut, during a visit to Craig Boyd Hairdressing in Leven PA General election: On the campaign trail Vans belonging to The Community United against Labour Party anti-Semitism group, Culpa, parked next to a police car ahead of the Labour Race and Faith Manifesto launch Getty General election: On the campaign trail Boris Johnson prepares to serve food at the Royal Welsh Showground PA General election: On the campaign trail Jeremy Corbyn tries his hand at bricklaying during a visit to the West Nottinghamshire College Construction Centre, in Ashfield PA General election: On the campaign trail Liberal Democrat leader Jo Swinson during a visit to the London design museum to discuss the party's tech policy PA General election: On the campaign trail Brexit Party leader Nigel Farage visits Seaham Getty General election: On the campaign trail Britain's Prime Minister Boris Johnson arrives for the Conservative party's manifesto launch in Telford Reuters General election: On the campaign trail Labour Party leader Jeremy Corbyn meets supporters during a visit to Thurrock in Essex PA General election: On the campaign trail Protesters wait for the arrival of Prime Minister Boris Johnson ahead of the launch of his party's manifesto PA General election: On the campaign trail Jo Swinson gestures from atop a crane during a visit to an eco home building site in Sheffield AFP via Getty General election: On the campaign trail Jeremy Corbyn meets a supporter on a train on his return from a visit to Sheffield PA General election: On the campaign trail Prime Minister Boris Johnson during a visit to Bassetlaw District General Hospital in Worksop, Nottinghamshire PA General election: On the campaign trail Nigel Farage campaigns in Hartlepool Getty Images General election: On the campaign trail Jeremy Corbyn outside an Amazon depot in Sheffield, to announce plans for a workers' rights revolution and to ensure big businesses pay their fair share of taxes PA General election: On the campaign trail Prime Minister Boris Johnson, Liberal Democrat leader Jo Swinson, First Minister of Scotland Nicola Sturgeon and Labour leader Jeremy Corbyn, during their successive appearances on BBC Question Time Leaders' Special at the Octagon in Sheffield PA General election: On the campaign trail Liberal Democratic Party leader Jo Swinson visits a primary school in Cambridge EPA General election: On the campaign trail Boris Johnson lays a brick during a visit to the Barratt Homes Getty General election: On the campaign trail Labour leader Jeremy Corbyn attends the launch of the party's election manifesto at Birmingham City University Getty General election: On the campaign trail Brexit Party leader Nigel Farage holds his party policy document during the party's policy launch in Westminster PA General election: On the campaign trail SNP leader Nicola Sturgeon joined Dundee East candidate Stewart Hosie to made a keynote speech at The Quay Getty General election: On the campaign trail Prime Minister Boris Johnson holds a part of an electrical appliance during a General Election campaign trail stop at Ebac in Newton Aycliffe AP General election: On the campaign trail Jeremy Corbyn poses during a visit to The Oatcake Boat on the Trent & Mersey Canal, Stoke Getty General election: On the campaign trail Jo Swinson and Willie Rennie Leader of the Scottish Liberal Democrats gesture, during a visit to Glasgow Reuters General election: On the campaign trail Prime Minister Boris Johnson during a visit to Jimmy Egan's Boxing Academy at Wythenshawe, while on the campaign trail ahead of the General Election. PA Photo. Picture date: Tuesday November 19, 2019. See PA story POLITICS Election. Photo credit should read: Stefan Rousseau/PA Wire Stefan Rousseau PA General election: On the campaign trail Liberal Democrat leader Jo Swinson (left) during a visit to the Gurdwara Singh Sabha Temple in Glasgow PA General election: On the campaign trail Labour leader Jeremy Corbyn meets Colin and his mother after a speech at the University of Lancaster Getty Images General election: On the campaign trail Britain's Green Party co-leaders Jonathan Bartley, Sian Berry, and deputy leader Amelia Womack, wave during the launch of the party manifesto at the Observatory at London Wetland Centre Reuters General election: On the campaign trail Scottish Nationalist Party leader Nicola Sturgeon meets voters and activists at Cafe Roma in Clarkston, Glasgow AFP via Getty General election: On the campaign trail Protestors participate in an anti-Boris Johnson rally in his constituency Uxbridge, west London AFP via Getty Images General election: On the campaign trail Brexit Party leader Nigel Farage looks out from a window on the Kestrel crabbing boat in Grimsby fish dock PA General election: On the campaign trail Prime Minister Boris Johnson on a tug boat in the port of Bristol PA General election: On the campaign trail Green Party Co-Leader Sian Berry has a selfie taken with a supporter at the Observatory, London Wetlands Centre PA General election: On the campaign trail Jo Swinson helps chef Leonardo Mastofilippo to make pizza during a campaign visit to cafe Amisha in South Bermonsey EPA General election: On the campaign trail Labour leader Jeremy Corbyn with WASPI (Women Against State Pension Inequality) ladies during a visit to Birkenshaw Sports Barn in Uddingston EPA General election: On the campaign trail Boris Johnson at the unveiling of the Conservative Party battlebus in Middleton, Greater Manchester PA General election: On the campaign trail Nicola Sturgeon sits alongside Cycling Without Age user Cyril Corcoran (centre), aged 78, in an electric tricycle during a visit to Hawick, in the Scottish Borders PA General election: On the campaign trail Nigel Farage holds a fish during a stop at the Grimsby Seafood Village PA General election: On the campaign trail Liberal Democrats leader Jo Swinson in the boxing ring at Total Boxer, a specialised boxing gym which offers training to young people as a means of keeping them away from violence, in Crouch End, London PA General election: On the campaign trail Britain's Prime Minister Boris Johnson reacts as he participates in a school art lesson, making a clay figure at George Spencer Academy, west of Nottingham AFP via Getty General election: On the campaign trail Labour Party leader Jeremy Corbyn during a visit to the Heart of Scotstoun Community Centre in Glasgow PA General election: On the campaign trail Scotland's First Minister and SNP leader Nicola Sturgeon visits the Cafe Gelato in Rutherglen, Glasgow EPA General election: On the campaign trail The Green Party launch in Bristol PA General election: On the campaign trail Farage enjoys some chips at the start of a nationwide tour for the 2019 general election, in Whitehaven AFP/Getty General election: On the campaign trail Labour leader Jeremy Corbyn unveils the Labour battle bus in Liverpool PA General election: On the campaign trail Boris Johnson reacts as he talks with workers whilst weighing packaged tea bags at the Tetley Tea Factory in Stockton-on-Tees AFP/Getty General election: On the campaign trail Brexit Party leader Nigel Farage on the campaign trail in Whitehaven, where he was confronted by Karl Connor PA General election: On the campaign trail Lib Dem leader Jo Swinson speaks to volunteer Rosie Squires in the Stainforth 4 All charity shop during a visit to Stainforth in South Yorkshire to meet people affected by flooding PA General election: On the campaign trail Nicola Sturgeon poses with candidates during the SNP general election campaign launch in Edinburgh Reuters General election: On the campaign trail Labour Party leader Jeremy Corbyn with Laura McAlpine, the party's candidate for Harlow PA General election: On the campaign trail Anti-Boris Johnson protesters demonstrate outside the hall where the Conservative Party were launching their campaign in Birmingham EPA General election: On the campaign trail Green Party Co-Leader Sian Berry (left), Deputy Leader and Parliamentary Candidate for Newport West Amelia Womack (right), and Bristol West Candidate Carla Denyer (centre) at the launch of the party's general election manifesto in Bristol PA General election: On the campaign trail The PM tastes whisky during a visit to Diageo's Roseisle Distillery near Elgin, Scotland Reuters General election: On the campaign trail Nigel Farage poses with boxer Dereck Chisora during a visit to a boxing gym in Ilford Reuters General election: On the campaign trail Jo Swinson visits a science class at Hinchley Wood School near Esher Reuters General election: On the campaign trail First Minister Nicola Sturgeon holds a guitar as she visits Dalkeith Community Hub Getty General election: On the campaign trail A man heckles as Jeremy Corbyn gives a speech at the Senior Citizens Hall in Macclesfield Getty General election: On the campaign trail Britain's opposition Labour Party leader Jeremy Corbyn visits residents affected by flooding in Conisbrough Reuters General election: On the campaign trail Boris Johnson gestures as he helps quality control staff during a visit to the Tayto Castle crisp factory in County Armagh, Northern Ireland AFP via Getty General election: On the campaign trail The Lib Dem leader eats a marshmallow during a visit to Free Rangers Nursery in Midsomer Norton, Somerset PA General election: On the campaign trail Nigel Farage visits a butcher's shop in Sutton-in-Ashfield Reuters General election: On the campaign trail SNP leader Nicola Sturgeon visits Blosson Tree children's nursery AFP via Getty General election: On the campaign trail Jeremy Corbyn laughs during a visit to the Scrap Creative Reuse Arts Project, Sunny Bank Mills Getty General election: On the campaign trail Prime Minister Boris Johnson hold Rosie the rabbit during a visit to a primary school while on the campaign trail in Taunton AFP via Getty General election: On the campaign trail Jo Swinson playing with children at the Battersea Arts Centre in Lavender Hill PA General election: On the campaign trail Farage addresses supporters at the Washington Central Hotel in Workington PA General election: On the campaign trail Jeremy Corbyn poses with Labour activists as they canvas in Govan, Glasgow PA General election: On the campaign trail Nicola Sturgeon during a visit to The Shed, a Climate Challenge community project at North Edinburgh Arts PA General election: On the campaign trail Prime Minister Boris Johnson meets six month old Willow Rose Anderson, at the Lych Gate Tavern in Wolverhampton PA General election: On the campaign trail Jo Swinson holds up a sack of barley during a visit to Crafty Maltsters Scotland Getty General election: On the campaign trail Jeremy Corbyn tries a scone in Bentley, Doncaster PA General election: On the campaign trail Nigel Farage takes a mouthful of homemade cheesecake during a stop at the Grimsby Seafood Village PA General election: On the campaign trail Boris Johnson hauling a consignment of frozen chocolate gateaux, during a visit to Iceland Foods HQ in Deeside PA General election: On the campaign trail Jo Swinson puts on a high visibility jacket during a visit to Sigma Pharmaceuticals, a family run pharmaceutical wholesaler in Watford AFP via Getty

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.

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