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(CNN) —  

The first game-changing moment of the 2019 UK general election could come on Tuesday evening, as millions of Brits tune into the first televised debate of the campaign.

Prime Minister Boris Johnson and Labour leader Jeremy Corbyn will go head to head in the ITV event, the first in an extensive schedule of televised showdowns between now and polling day.

It marks an opportunity for Corbyn, currently trailing Johnson in opinion polls, to claw back the gap between him and his rival.

But the event won’t feature Liberal Democrat leader Jo Swinson, whose party has taken legal action against the broadcaster over the snub.

Here’s what you need to know ahead of Tuesday’s debate.

What is the format of the debate?

Tuesday evening’s debate will take place at 8 p.m. (3 p.m. ET) for one hour, and will be moderated by ITV presenter Julie Etchingham.

Only Johnson and Corbyn will take part – the first time leaders of Britain’s two major parties have faced each other head-to-head on the same stage.

A series of questions on Brexit and domestic policies is expected, though ITV has kept most details of the debate under wraps.

Six party leaders -- and the Conservatives' stand-in Amber Rudd -- during the 2017 debate.
WPA Pool/Getty Images
Six party leaders -- and the Conservatives' stand-in Amber Rudd -- during the 2017 debate.

How important will it be?

The event gives both Johnson and Corbyn their first opportunity to make their election pitch to a major audience. Previous debates in the UK have attracted more than 7 million viewers, representing a significant portion of the country’s electorate.

For Corbyn, the spectacle is a chance to turn around polling that shows him trailing Johnson. Having the Labour leader alone on stage alongside Johnson is already something of a victory for the party, allowing them to sideline the Liberal Democrats and helping hammer home their point that Corbyn and Johnson are the only two candidates who could become prime minister after next month’s vote.

The Prime Minister’s team, meanwhile, will be desperate to avoid a mistake from Johnson – well-known for his free-wheeling and improvisational style – that could dominate Wednesday’s front pages.

And after a campaign season in which no group has broken through, the event also gives both parties the opportunity to pick up some much-needed momentum heading into the busy period in which each group unveils their manifestos.

What role have previous debates played?

Britain is relatively new to televised election debates, having only held its first in 2010 – but the broadcasts have already played pivotal roles in recent elections.

In the country’s first ever debate in 2010, a strong performance from Liberal Democrat leader Nick Clegg provided a monumental boost to his party’s campaign that even saw Britain’s traditional third party lead a handful of opinion polls for a few days.

Clegg’s answers prompted then-Prime Minister Gordon Brown to repeatedly say “I agree with Nick,” which was predictably capitalized upon as a Liberal Democrat campaign slogan.

Five years later, reluctance from David Cameron’s Downing Street to take part in head-to-head events resulted in a cluttered seven-leader debate. Labour leader Ed Miliband was unable to lift his party’s fortunes during the event, and eventually suffered a surprisingly comfortable defeat to Cameron.

But in Britain’s most recent election in 2017, the TV debates played a crucial part. Theresa May, desperate to avoid a slip-up that would erode her comfortable lead in the polls, refused to take part in the only scheduled debate between seven party leaders, sending Cabinet member Amber Rudd in her place.

But May’s no-show dominated discussions and ensured she lost the debate before it had even begun. The decision encapsulated a woeful Conservative campaign defined in part by the Prime Minister’s refusal to engage with the general public, and the group ultimately suffered a stunning loss of seats after throwing away a substantial polling lead.

Who isn’t taking part?

The debate won’t feature Liberal Democrat leader Jo Swinson, a decision that the party has reacted furiously to.

Her exclusion means Brits won’t hear from any parties opposed to Brexit – though the Labour Party is advocating a second referendum on a deal. Swinson described the decision as “an establishment stitch-up that shuts out the views of millions of people in the country.”

Along with the Scottish National Party, the Liberal Democrats launched a legal challenge arguing that the broadcasters were breaching impartiality rules.

But on Monday, London’s High Court rejected their case, so the debate will go ahead without them as planned.

When are the other debates?

Johnson and Corbyn will go head-to-head again in a BBC debate on December 6, just six days before polling day.

Before then, there will also be a Sky News debate between Johnson, Corbyn and Swinson on November 28. And just in case the British public wanted even more debating, there’ll be two seven-leader debates: one on November 29, just one day after the three-headed event, and another on December 1.

That means there’ll be three debates in the space of four days at the end of the month – though some of the major parties may decide to send a surrogate to one of them.