Rory Stewart has said what many Conservative moderates are thinking about Boris Johnson

The international development secretary's vow is a blow to a future Johnson government - and could prevent one coming about to begin with. 

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Rory Stewart has become the most senior Conservative to say that he will not serve in a Boris Johnson government, after the current frontrunner said that he would take the United Kingdom out of the European on 31 October, deal or no deal.

It’s an obvious blow for Johnson in that Stewart is a rare complete politician – he’s respected on Whitehall as a competent administrator who can run a departmental brief effectively and performs well on broadcast and in the House of Commons. One reason why governments tend to struggle later on in their life is that they tend to have a surfeit of good departmental managers and a shortage of good political performers. (This is also why new governments tend to have a reshuffle fairly early on, when it emerges that a bunch of the people who were very good at opposition are pretty bad at running things.)

But the bigger problem for Johnson is that while few Conservative MPs have the political courage to say publicly that, having been told by the former Mayor of London that he would not pursue a no deal exit they cannot trust him. But most are privately of the view that the Johnson train is leaving the station and it is better to be on it than under it, at least in public.

The trouble though is the ballot to decide the final shortlist of two to face the members is a private ballot of MPs – and while the number of Conservative MPs publicly echoing Stewart is non-existent, the number who agree with him isn’t.

A few months ago we thought that the place to be in this leadership election was to be impeccably pro-Leave, trusted by former Remainers and not called Boris Johnson, and that this candidate, whether they were called Penny Mordaunt, Andrea Leadsom, Dominic Raab or Graham Brady, would ultimately make it to the final two partly through their own popularity and partly because they were a necessary device by anti-Johnson MPs to thwart Johnson.

But the terrible polls have seen that space collapse, Mordaunt isn’t running, and Raab has, in the eyes of that caucus, been revealed to be an ideologue who raises similar objections to Johnson himself. But it could be that Johnson’s decision to commit himself on no deal means that the “Leaver but not Boris” space once again becomes the winning place to occupy in the parliamentary party, whether the beneficiary of that is Graham Brady, Andrea Leadsom or someone as yet unknown.

Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics.