he did probably avoid making the rift with those voting against him even bigger. By not playing an active role in the debate, he will take some flack for failing to carry the majority of his party, but in terms of party discipline and morale, that might be preferable over him playing an active role and really antagonizing the majority (or, as the case might conceivably have been, large minority) that voted against his position.

So I'm not sure if you can actually call it a mistake. He's set the whole thing in motion and can rest assured that he'll be given due credit for it passing, even though most of the votes came from the LibDems and Labour; he could afford to take a step back at the moment supreme and avoid irritating the "rebels" in his party further.