Andrew Bowie, Theresa May's parliamentary private secretary, has endorsed Boris Johnson for the Conservative leadership. The frontrunner's camp believe it is a significant coup, and in two respects they are right.

That Johnson has won over the current prime minister's closest Commons ally reflects the fact that Tory MPs of all persuasions increasingly believe that this election is no longer an exercise in choosing their preferred leader. The question has changed. Instead, they are deciding what sort of accommodation they would like to reach with the former foreign secretary in the event of the victory most Conservatives, including his rivals, have concluded is inevitable.

In that respect Bowie's endorsement is noteworthy but not especially so. Plenty of MPs who at one time or another have worked closely with May, and remain personally and politically loyal to her, have endorsed Johnson. Take the ministers Michael Ellis, her longtime PPS at the Home Office, and James Brokenshire, one of her proteges. Both are supporting Johnson, and for that matter declared much earlier in the day than Bowie, who transfers his allegiance from Matt Hancock.

What makes Bowie's endorsement rather more significant is the fact that he is one of the 12 Scottish Conservative MPs first elected in 2017. He joins Colin Clark, who unseated Alex Salmond, and Ross Thomson, a hard Brexiteer, in endorsing Johnson. Five others - Kirstene Hair, Luke Graham, David Duguid, Bill Grant and Stephen Kerr - are backing Michael Gove, as is Scottish Secretary David Mundell. Jeremy Hunt and Rory Stewart have one MP apiece, as did Mark Harper before his elimination. The twelfth member of the 2017 intake, Alister Jack, is a whip and thus cannot declare.

Gove, then, is the clear leader among his compatriots. He will remain so even if Douglas Ross, who voted for Harper in the first round, declares for Johnson tomorrow. Look at it another way, however, and the picture is rather less rosy for the environment secretary. Of the three Scottish Tory MPs who initially backed Matt Hancock, Gove has won the support of only one: David Mundell, who as one of the cabinet's most vociferous opponents of a no-deal Brexit was hardly going to back the favourite. The remaining two - Bowie and Paul Masterton - have broken for Johnson and Rory Stewart respectively.

That stark difference is noteworthy because every Scottish Tory MP entered this race with the same primary objective: backing the candidate best-placed to protect the Union. Yet Bowie and Masterton have reached opposite conclusions as to who that candidate is.

To understand both stances you first have to understand the overall complexion of Johnson and Stewart's respective coalitions. Members of the government payroll make up a sizeable chunk of Johnson's. In backing him, the ministers and bag-carriers in question have signalled they would quite like to stay part of it. Bowie is among them. Many of Stewart's supporters have government jobs too, it is fair to say that most of them either wouldn't be invited to serve under Johnson, or wouldn't accept an invitation. Masterton quit as a PPS to vote against no-deal and so falls into this category.

But it isn't desire for preferment alone that has driven Scottish MPs to support Johnson. Nor will it be the only thing that entices those who do not support him now into his government. Rather they have made the calculation that, given the likely inevitability of his victory, that the best way to minimise the potential risk to the Union posed by a Johnson government dominated by English Eurosceptics is by wielding influence from inside the tent. The real risk, they argue, would be ceding that degree of control.

So even though the frontrunner is the first choice of only two Scottish MPs, most, if not all, could live with the increasingly likely fact of his premiership. Even those who are implacably opposed believe that the risks to the Union could be mitigated with some strategic thinking from Johnson and Ruth Davidson. It is fundamentally a disagreement over means rather than ends. And the divergent paths Bowie and Masterton have taken reveal that Scottish MPs see the fundamental choice on protecting the Union much as their English and Welsh colleagues do on Brexit: moderate a Johnson government from the inside, or constrain it from the backbenches. Their uniquely Scottish concerns have found a common British expression.

There is a big loser from this dynamic: Sajid Javid. Despite winning the endorsement of Ruth Davidson, not a single Scottish Tory is supporting him. Privately, his team admit surprise and disappointment. But illuminates a truth that the next prime minister would be well served to remember: the 13 Scottish Conservative MPs cannot be reliably swayed as a bloc.