There is a buoyancy about the Labour campaign at the moment, his public performances have improved, and his team believe he has more to gain than to lose from taking part in this big set-piece campaign event.
And for today at least, it gives Labour a tactical advantage, ammunition to portray Theresa May as cowering and afraid of making her arguments.
Tory sources say there is no chance that she will change her mind and join him in the last minute dash to the debate.
One source said: "This election isn't about who can squabble with six other politicians - it's about who can negotiate against 27 other nations."
But it makes her vulnerable to the charge that she is evading scrutiny, despite the fact that, like the other leaders, she has been taking regular questions from journalists and will take part in another big public Q&A on Friday night.
It's the direct head-to-head debate with Jeremy Corbyn that she, just like David Cameron in 2015, has refused to take part in.
Exactly as in the wider election campaign, the incumbent, the frontrunner has everything to lose, the insurgent opponent, everything to gain.
Earlier I wrote that Jeremy Corbyn had nothing to lose in taking part in tonight's big debate, he is clearly enjoying being the insurgent in this campaign, with some of the polls tightening and his message getting a wider hearing.
He has indeed, a lot to less to lose than the incumbent, Theresa May who is the frontrunner.
But Labour sources also suggest he had an additional motivation.
I'm told that Mr Corbyn and his team wanted to move on from a day of damaging headlines about his lack of grip on his childcare policy, "quick fast".
His decision to take part after weeks of refusing to do so has certainly changed the subject.
That's denied by sources close to the Labour leader, who say their feeling from Tuesday morning was to take part, but logistical issues meant that the team and Mr Corbyn weren't able to make the final decision until this morning.
After flirting with the idea for the last few weeks, Jeremy Corbyn's confirmed he will take part in the seven-way party leaders' debate tonight in Cambridge.