Now he is seen as an ultra-loyal model Blairite - pro-Europe, anti-drugs, pro-PFI for hospitals.
But he was recently thrilled to see his son, albeit a Blairite, win a seat there.
Blairite journalists have written earnest columns stressing the emblematic nature of the opening of the new gallery.
He might be Gordon Brown's guy now, but Lord Mandelson is clearly still a Blairite at heart.
As usual, this restatement of the Blairite creed is both admirable and maddening.
Watch out too for the ultimate Blairite, Peter Mandelson, currently Minister without Portfolio.
And that would make the Socialists even less likely to embrace Blairite reform.
Downing Street's promotion of the Blairite Alun Michael over the popular Rhodri Morgan?
If the new mayor has real power, old Labour politicians will see him as a Blairite threat to their fiefs.
The Labour Party had been traumatised by successive defeats and swallowed the Blairite medicine as its only hope of resurrection.
This stress on the need to create a broad progressive movement has two main consequences for the Blairite approach to government.
But incumbency may still enable some Tories to cling on to seats which otherwise would have been swept away by a Blairite tide.
Often spoken of as a possible Blairite challenger to Mr Brown, he decided against standing for the Labour leadership when Tony Blair quit.
Mr Johnson was an obedient Blairite, and has recently advertised his enthusiasm for electoral reform, but that scarcely amounts to a distinct political philosophy.
Nor is it an attempt to win newcomers over to a particular world view - the guests are not being forcibly injected with Blairite nano-particles.
The split is evident in Labour's frontbench education team, which is led by the Blairite Stephen Twigg but also contains the more sceptical Lisa Nandy.
But France represents a large potential obstacle to these Blairite goals.
Last week a Blairite guru, Anthony Giddens, he of the Third Way, who runs the London School of Economics, was in Madrid for an Aznarian hobnob.
They were doubtful about going to war, but, after weeks of Blairite persuasion, a big majority against in the opinion polls turned into a majority in favour.
Two or three weeks ago, with only a few exceptions, the cabinet and rising young Blairite ministers were determined to make the best of Mr Brown's near-certain succession.
That puts him firmly in the revisionist camp, rewriting the Blairite narrative of choice, diversity and independence, according to which all schools will improve if they face competition.
As Blairite ministers acknowledge, raising the state-pension age had been taboo even though it was originally set at 65 in 1925, when pensioner life expectancy was much lower.
But lest people get carried away with visions of smiling picnickers in the new green arcadias of Blairite Britain, the urban parks scheme come with a very nineties twist.
"Renzi is seen as a Blairite, but it's never been a Blairite left in Italy, and I don't think he was trusted by some of the party faithful, " Andrews told CNN.
You now hear the most Blairite of the education ministers privately berating Conservatives for not making every school a grant maintained school before Labour could get in to dismantle the system.
BBC: NEWS | UK | UK Politics | In full: George Osborne speech
"I personally believe that Brown is best placed to lead us forward but that's only on the basis that he walks away from the Blairite policies of the past", he told me.
These political and systemic differences may explain why one Blairite flunkey was heard to explain recently that in Europe there will be a set of third ways, not a single third way.
Now the Tories are split between those who think power will come from elbowing Labour aside in the centre-ground and those who think they should eschew anything Blairite and seek to be distinctively Conservative.
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