It is easy to see the thinking behind Brown’s original plan. Brown could frame the challenge as a chance for Cameron to prove that he was not all style and no substance. By taking the initiative, Brown could set the terms of the debate—eg how many there would be, what the topics would be and the like.
Certainly, there would have been a media clamour for Cameron to say straight-away whether or not he accepted Brown’s invitation. It also would have dominated coverage of Brown’s speech, making him appear bold and confident.
One wonders what Brown will do now. The fact he was planning to challenge Cameron to debate shows that he accepts that he is, to use Mandelson’s phrase, “the underdog” in this coming campaign. Brown still needs a chance to go head to head with Cameron as it is one of the very few potential game changers there is left. So, I wouldn’t be that surprised if Brown still ends up issuing this challenge.