"It's all about leadership now" - it is not, any longer, according to that particular diplomatic source, about the finer details of customs posts; the never-ending tangle of whether it's UK or EU law that's supreme; or whether a sausage that's been made in Bolton needs to be inspected if it is going to be sold in Belfast.
A deal on the Northern Ireland Protocol is so close that the negotiating teams have been consulting the thesaurus to help pick a name, in scenes reminiscent of the '80s political comedy, "Yes Minister".
Sorting out the protocol - those post-Brexit trading arrangements - matters practically if you live in or do business with Northern Ireland.
And it matters symbolically a great deal for Rishi Sunak and the government, eager for this bitter hangover from the Brexit negotiations to fade.
The agreement, whatever it ends up being named, is broadly done, and likely to be unveiled on Monday.
One Whitehall source says there's been a "very disciplined approach to solving the problems"; now an arrangement has been struck that "unambiguously works" after weeks of working through the issues.
But that painstaking practical negotiation could come to naught if it's not matched with political force.
It's in the constitution of some ranks of the Conservative backbenches and some parts of the Northern Irish Unionist DUP to be suspicious of what emanates from any group in charge in Number 10.
Add in the fact that many on the Eurosceptic wing of the Tory party are, politely put, not natural supporters of Mr Sunak.
Some, like the former leader Iain Duncan Smith and the DUP's Sammy Wilson, have been setting tests for the deal before they know for sure what's in the final version - easier perhaps then to cry foul later on.
It is rarely possible to please all of the people all of the time. But on the issue of Brexit and Northern Ireland, it is near impossible.
A diplomat says, "the government simply needs to push it forward, knowing that part of the DUP will not be pleased" - so the question is whether Mr Sunak's push to get a deal through is stronger than the resistance it will inevitably find along the way.
Number 10's handling of the matter so far could make that harder. Mr Sunak is not the first prime minister to preside over a "will they, won't they?" phase during talks with the EU.
But while a deal's been in the offing, there's been something of a vacuum, giving space for critics to opine.
Notably it's given time for Boris Johnson to pile in too, making clear in the last few days that his support won't come easy. He's still a totem for Brexit, and is becoming a rallying point for Mr Sunak's critics.
As the Whitehall source says, "the key dynamic which is emerging is that Boris is throwing himself to the forefront of the opposition" - that won't make the prime minister's life any easier.
Irony alert - remember that Boris Johnson was the prime minister who signed up to the Northern Ireland Protocol in the first place.
And it's worth noting that the deal thought to be on offer concedes far more ground to the UK than was thought possible then.
In fact, one member of Theresa May's team told me they would have "bitten your arm off" for the kind of arrangements that are now in play.
There is a likely acceptance of different customs routes, "green lanes" and "red lanes" for goods heading from Great Britain to Northern Ireland; it's likely too the EU has given ground on state aid.
But like it or not, for Rishi Sunak's Number 10, when his old boss speaks, many Conservatives and Brexiteers listen.
Don't be surprised to see, in the coming days, heated political arguments contrasting the agreements that Mr Sunak has reached, and the Northern Ireland Protocol bill that Boris Johnson's government introduced.
That bill is making its way through Parliament now and would, controversially, give the UK government the power to ignore the treaty that's already signed into law.
And don't underestimate the strength of feeling.
One former cabinet minister told me the expected deal would "let down the die-hard leavers", leaving the party vulnerable to attacks from the right, and would "split the Conservatives".
Unless there is a "miracle surprise", the same source cautioned that they and many of their colleagues won't back the deal.
The options for Mr Sunak then are that the deal passes (if there is a vote) "grudgingly", or if it falls, "it leaves his authority in tatters".
It is even, they suggest, a "conceivable option" that the government ends up falling apart if the prime minister tries to ram it through. All this, of course, is happening in the context of a PM stuck way behind in the polls.
There was also disbelief in some quarters at the suggestion that the EU president would appear alongside the King to help boost the chances of a deal.
Buckingham Palace, as a rule, tries to appear beyond politics, well above the fray.
Yet there was the suggestion that Ursula von der Leyen would meet publicly with the King this weekend at Windsor Castle, and even that the deal itself might be given the title the Windsor Agreement - to "throw a bit of glamour" around the closing stages, a source suggested.
But I'm told the European Commission disapproved. Downing Street says officially that the reason the visit was called off was "operational".
Whatever the whole truth, it's made for a messy 24 hours since our colleagues at Sky News broke the story. And it met with disapproval in the very DUP circles that the prime minister needs to get on board.
The Unionists are deeply committed to, and proud of, the monarchy. Arlene Foster, the former DUP leader, was rarely seen without her glittering brooch in the shape of the crown. But the impression that Downing Street was trying to play the Palace into the process has rankled.
The DUP former deputy leader, Lord Dodds, said, "to plan for politicising the monarchy in this way is very serious and reinforces the questions about No 10's political judgement over the protocol".
That doesn't sound like a political leader in the mood to play nice.
We know that Rishi Sunak has been able to find a way to strike an agreement with the EU to help ease the problems that stemmed from the special arrangements for Northern Ireland.
We know Number 10 has been willing to spend political energy and effort getting this far.
But once the black and white of the deal emerges into the light of day, political guile, presentation and force may be required to drive it through.
So far in Downing Street, Rishi Sunak has tried to avoid fights with his party. This time, on this most fraught of issues, an almighty argument awaits.