It is understandable that the Tories are keen to capitalise on what appears to be a split in Labour’s high command over whether Brown would be game or not. The Tories are also keen to highlight the Cameron Brown contrast, one that undoubtedly plays to their advantage. I also suspect that they rather hope that in a debate, Brown’s temper and his evident disdain for Cameron would boil over and drag Labour down even further.
But if Brown has any political nous left, he’ll say yes to the debate. . If Brown is going to win the next election he needs a dramatic game-changing moment, a head to head debate is one of his best chances of producing one and Cameron is now too committed to back-out. Also, the expectations for Brown are so low that if he gets through it without losing his temper or saying something like zero-percent increase then he’ll have got over the bar.
If Brown ducks Cameron’s challenge, it will show that he cannot accept the new political reality that means that unless something changes he is going to lose the next election. I expect that behind the scenes, people are urgently trying to make Brown adjust to this reality, telling him that he is "the underdog" now.
PS If there is a debate, I’ll wager that Brown will try and deal with the temper issue in the same way that Sarkozy did during the French presidential debate, remaining calmer than normal and then accusing his opponent of losing their temper.