Given that Tony Blair ousted John Major from power in 1997, they are hardly ever likely to be best buddies. But for both prime ministers, Northern Ireland was a big priority, and in the progress to peace there is one of the best examples of what boldness and courage, political commitment and hard work, can do.
So as the 23 June EU referendum nears, it is good for the campaign that they went there together, and that their voices are being heard on a subject on which all too little has been spoken, namely the potential impact of a Leave vote on Ireland north and south.
Border controls in Ireland have a resonance unlike no other in these parts. We in Great Britain have not had to live with the kind of checks that the security situation – the smuggling and the organised crime related to terrorism – demanded.
Nobody from Leave has been able to explain how they can meet their promises to curb freedom of movement by EU nationals without strict border controls returning. Nor have they been able to explain how they would make up for the damage done to both UK and Irish economies by the departure from the Single Market on which so many of our jobs, and so much of our prosperity depend.
So the Major-Blair duo has been drawing attention to the possible return of controls at what is the only land border between the UK and the EU. Add in the possibility on the other side of the UK, South East England, of the border at Calais moving back to Kent (another issue on which Leave are conspicuously quiet) and you start to get into the nitty gritty of just two of the many difficult and unintended consequences of Brexit.
Nobody from Leave has been able to explain how they can meet their promises to curb freedom of movement by EU nationals without strict border controls returning.
As with everything said by people spelling out inconvenient truths about Leave, they have been quick to dismiss the claims. If an expert opinion conflicts with their own, that is what they do. Only they speak common sense, apparently, because they are on the side of the people. Yeah right.
Of the many big lies told by the Brexit Lie Machine few come bigger than the lie that the Leave campaign is speaking up for the little man against an elite. Let's just examine for a moment who these leaders are.
Boris Johnson Eton. Oxbridge. Telegraph. Tory MP. Mayor of London.
Michael Gove Oxbridge. Murdoch press. Tory MP. Now in a post that requires him to dress in clothes rarely seen in working men's clubs.
Rupert Murdoch. A billionaire Australian born American who uses his papers in Britain to help promote his own personal, political and commercial interests.
Paul Dacre. A multimillionaire who pockets plenty in EU grants for his vast Scottish estate.
And at the Vote Leave top table we have people who have spent much of their careers trying to undermine the NHS now claiming to be its great defenders.
In a Trumpian world where a multimillionaire son of inherited wealth can become the standard bearer of the man and woman crushed by the crash, we should perhaps not be surprised at the con these people are pulling. But we should, given what is at stake, at least be aware of it.
Because the turning of the concept of 'expert' on its head has been a truly remarkable part of the con trick at the heart of Leave. 'Expert' used to mean someone who knew what they were talking about. So when nine out of ten economists argue strongly for Remain, and when every major economic institution bar none comes out against Leave – warning of potentially cataclysmic consequences – these are all dismissed by the aforementioned elite as pro-establishment Project Fear flunkeys.
Sometimes, when you have difficulty making a decision, expert opinion is the one worth listening to. When it comes to the peace process, its strengths and weaknesses, you don't have many greater experts than Tony Blair and John Major. If leaders have a genuine fear that a course of action will have bad consequences for something as important and at times as fragile as the peace process, they have a duty let alone a right to say so, however much the Project Fear attack goes up.
When it comes to the peace process, its strengths and weaknesses, you don't have many greater experts than Tony Blair and John Major.
There is a lot to be scared about. John Major also warned of the danger of a second referendum on Scottish independence if the UK votes to leave and, as I have said before, I think in those circumstances there could be a different outcome.
So yes, the economy is on the ballot paper on June 23, and the Single Market and the three million UK jobs that partly depend on it should be argument enough to stay. But the peace process is on the ballot paper too. And so is the Union.
They are the kind of issues too serious to be sacrificed on the altar of Boris Johnson's ambition to follow in the footsteps of Major and Blair into Downing Street. I hope today is one of those days when people really stop and think what is at stake.
I know, from my regular visits to Ireland, that the business community there is, if anything, even more united for the UK staying than business is here. Today Ibec, a group representing Irish business, is launching a poster campaign at Dublin Airport, aimed at the many UK voters, and Irish voters with UK connections, passing through there daily. The message is simple: 'DON'T GO. Let's work together.'
While Leave claim new trade deals will be worked out quickly and with ease, anyone with any experience of actually doing them knows how untrue that is. We are set, if we leave, for years of uncertainty and with it massive economic risk.
Ibec CEO Danny McCoy says: "If the UK votes to leave, not only will the UK economy suffer, Ireland will also be badly affected. An EU without the UK would be a lesser Union.
"A UK exit would send Ireland, Britain and Europe into uncharted and treacherous waters. The value of sterling has already fallen significantly, a vote to leave would prompt a further significant depreciation, heaping pressure on businesses trading with the UK. This is in addition to the countless other risks that would arise during and after the period of a negotiated exit.
"A UK departure would be a blow to the Irish recovery and result in a protracted period of uncertainty. It would undermine Europe's ability to act collectively and decisively in the world and would push the EU back into a damaging period of crisis management, at a time when it should be looking to the future."
While Johnson, Gove, Farage et al blithely claim new trade deals will be worked out quickly and with ease, anyone with any experience of actually doing them knows how untrue that is. We are set, if we leave, for years of uncertainty and with it massive economic risk.
In this era of disbelief, and of anti-politics, anti-business and anti-expert, it is easier than it should be for a Trump or a Johnson to gain traction. But sometimes it really is worth listening to people who know what they are talking about. Love them or hate them, Major and Blair are seriously worth listening to on the fragility of the peace process. And the overwhelming volume of the expert voices warning of economic calamity are worth heeding too, because the prospects of it are all too real.
Alastair Campbell is a British journalist, broadcaster, political aide and author, best known for his work as Director of Communications and Strategy for Prime Minister Tony Blair between 1997 and 2003. He is the author of two books on mental health and is an ambassador for Time to Change.