It will be the biggest moment of the campaign so far. On Tuesday night, Boris Johnson and Jeremy Corbyn will go head to head in an ITV debate.

I say in The Sun this morning that this debate is the biggest risk that Boris Johnson has taken in this election. There’s a reason why none of his predecessors as PM agreed to such an encounter.

But Boris Johnson’s team calculated that this debate was worth it as it enables them to frame this election as a choice as to whether you want Boris Johnson or Jeremy Corbyn as PM. He has a 22 point lead on this question and asking it brings 2017 Tories who have gone over to both the Liberal Democrats and the Brexit Party back into the fold.

Tuesday night’s format should favour Boris Johnson. There’ll be half an hour on Brexit and then half an hour on domestic issues. Given how convoluted Labour’s Brexit position is, the audience’s patience with Corbyn may well have run out before the debate moves on to domestic policy.

The danger for Boris Johnson is that Jeremy Corbyn uses Tuesday night to establish himself as the only person who can stop Boris Johnson from spending Christmas at Chequers. Right now, the Tories are benefiting from a split opposition. If Corbyn can use Tuesday night to squeeze down the Lib Dem vote then that’ll make it that much harder for the Tories to win a majority.

Corbyn could also use the debate to force his opponent to defend the cuts of the past nine years. In this campaign, Boris Johnson has been talking more about his record over the past 100 days rather than the Tory one over the last nine years. If Corbyn can make Boris Johnson defend  ‘austerity’ he will dent his appeal in the Labour seats that the Tories need to win a majority. Those helping prepare Boris Johnson for the debate expect Corbyn to lean heavily on people’s personal experience of the cuts to try and force Johnson to engage on these questions.

If Boris Johnson bests Jeremy Corbyn on Tuesday night, he’ll still be on course for the majority he needs to break the Brexit logjam. But if he stumbles, then that’ll give Corbyn the opening he craves.