Martin Kettle in the Guardian writes an excellent summary of Cameron and Grammargate.
He points out that, in order for Grammar schools to be Cameron's "Clause 4", he has to win "it". "It" is rather difficult to win if there is no party ballot on it, as there was for Clause 4 with the Labour party:
Cue a "clause 4 moment"? Sort of. But don't forget the whole point of a clause 4 moment. You have to win it in order to have one. Blair confronted a totemic anachronism and won. Cameron now finds himself doing something similar but on a messier and more contentious issue without a defining ballot to cement his victory. Without that proof he will always be vulnerable to the "more show than substance" charge.
Kettle is in no doubt that this is a defining moment for Cameron:
No one can yet say with certainty whether Brown will be a more formidable adversary than late Blair or a weaker one. One must also beware of assuming that Blair's triangulation strategy in the 1990s is the only one worth replicating for Cameron. But Cameron's strategy is now delivering lower ratings than it was. He is more vulnerable to his rejectionists than before. This is a pivotal moment. Cameron knows it. His party knows it. And, most important of all, Brown knows it too.