So Jeremy Corbyn has well and truly nailed his colours to the anti-grammar school mast.

When he sat down at PMQs last week to cheers from behind him after grilling Prime Minister Theresa May over her plans to bring back selection, he could reflect that he had hit upon the one issue that would bring his backbenchers into line.

And then this week at Labour conference, to yet more cheering, he set out plans for a Day of Action this Saturday to rally the Labour troops on the issue and ramp up opposition among the public.

This, after all, is the man who, it is widely reported, divorced his first wife because she was keen to send one of their children to grammar school.

So, good news for opponents of the government’s Green Paper on schools? Of course they want as broad alliance as possible, with as many activists as energised as possible.

Well, maybe not.

Whisper it, but there are a growing number of voices (from the left as well as right) who worry that Corbyn’s unconstrained love for the anti-grammar effort could be disastrous.

Some of these have even been raised on Twitter, but more on what are known as the “backchannels”.

These concerns centre on the boring subject of parliamentary mathematics. Until JC’s recent interventions there was a growing cross-party alliance featuring nearly all opposition MPs and a small but vocal number of Tories (reckoned to be around 30 and including former education secretary Nicky Morgan) - enough to defeat Ms May’s plans in the Commons.

But with Mr Corbyn now making the issue central to his re-elected leadership, suddenly it becomes much harder for Tories to side with the opposition troops. The argument will be put to any potential rebels by the government’s political enforcers (known as the whips), a bit like this: “Do you really want to be seen helping to give the embattled Labour leader a famous victory? Do you really want to help him unify his divided party?”

Of course there will be other ways to force their arm - including the promise of career rewards from Downing Street. And, as one veteran Westminster watcher put it to me, concerns about strengthening the opposition would have been much more pronounced “if it were 2014, and we were talking about Ed Miliband”.

Then there is the House of Lords. Ms May may force legislation through the Commons, but the Lords (which is way more independent-minded) would more than likely be another matter.

But nonetheless the fact is that right now there is more than one leading figure in the resistance to more selection (including from the left) who really wishes that the leader of Her Majesty’s Loyal Opposition would stop banging on about grammar schools.

And that’s kind of counterintuitive when you stop to think about it.