They didn't leave the Labour Party for the newly formed SDP - though many of them would have done if Tony Benn had won the deputy leadership in 1981 - and they didn't trim to the left. They were, with a small s and small d, social democrats. They were in favour of Europe. They were against unilateral disarmament. They were, essentially - to borrow the abusive label attached to Hugh Gaitskell and his supporters by the Bevanite left in the Fifties - revisionists.
It's a disparate group, which included the late John Smith and now spreads from Roy Hattersley to Peter Mandelson. But as a tendency, it is most typified in today's Cabinet by the presence of George Robertson, the Secretary of State for Defence, who expects to be named today as the new Secretary General of Nato. Robertson wasn't always on Labour's right, revisionist wing; having grown up in Dunoon in the Sixties in sight of Holy Loch he would hardly, even as a policeman's son, have been human, young and to the left had he not demonstrated against the independent nuclear deterrent. "Och, och," we used to sing in CND, "there's a monster in the loch... we dinnae want Polaris." But he changed his mind far earlier than most of his current colleagues. He became Labour's main - and for a while almost only - representative within the foreign policy establishment. And he learnt to love the bomb (or, if not to love it, at least accept it; he always maintained that he didn't change his mind about the desirability of disarmament, only about the means of achieving it).
George Robertson, therefore, could hardly have been a more suitable candidate for the job of Defence Secretary. The military knew that he (and his department, in Labour terms the most right-wing in the Government) believed in them. And in so far as these judgements can be reliably made from the outside, he's been a real success. Having been tipped frequently as a future Foreign Secretary, he was upwardly mobile. Robertson may not be the most charismatic of politicians, but, in the daily televised news conferences during the Kosovo war, his solid, reassuring presence, and his "granite accent", as a positively gushing Le Monde profile called it yesterday, carried weight on both sides of the Atlantic. His Strategic Defence Review is now being talked of freely elsewhere in Europe as a model for adapting defence capabilities to post-Cold-War needs. He showed that, without any experience, he could manage one of the vastest bureaucracies in Whitehall.
He deserves, finally, to be well-satisfied that a Mori poll last week showed what would have quite recently been unthinkable, that Labour is now ahead of the Tories on defence.
All of which poses the question of why he is now abandoning a career in domestic politics to go to Brussels. My guess is that this was a difficult decision not only for Robertson, but for Blair as well.
Washington appears to have made it clear to London, at least after Rudolf Scharping, the German Defence Minister, dropped out of the picture, that it would back a Brit - but only if the candidate were a senior politician with a ministerial record. It may be a testament to the struggle Britain has in exercising influence from outside Euroland that most of the other big international jobs in Europe have been mopped up by Germany, France, Spain and Italy, and that the one it has secured needed American help; but it is equally a testament to Blair's internationalism, and his restless desire to exercise that influence, that he was, in the end, prepared to lose a good minister, risk a by-election defeat by the Scottish National Party, and send Robertson to Nato.
For Robertson, even more, there must have been grounds for hesitation. In countries with proportional electoral systems, and therefore party lists, it is quite easy for politicians to pass regularly between domestic and international jobs. The first-past-the-post electoral system, and the need to find a vacancy in a constituency with a warmly disposed local party, makes it much more difficult here. Difficult but not impossible. Chris Patten might have gone back into the Commons after governing Hong Kong. Moreover, he could well come back via the Lords. Tony Blair's first Leader in the Upper House, after all, was Ivor Richard, who had been both an EU Commissioner and an ambassador to the UN. Now 53, he could also, in theory, succeed Donald Dewar as First Minister of Scotland - where he will keep his home, commuting to Brussels weekly. In time he could well be in line for another big international job. But there is still a real element of risk.
Equally, you can see the attractions. It's an unusual feature of the current Cabinet that there is something of a log jam at the top. Most Cabinets have a sprinkling of older or obviously retirable politicians in the senior ranks who ensure a steady turnover. But with the big three offices of state plus the deputy premiership filled by politicians who cannot easily be moved around, let alone out, there is more of a problem, as last week's reshuffle rather emphasised. Robertson could have hung on, in the hope, rather than certainty, of replacing Robin Cook at some point after disappointing Blair by refusing the Nato job. Or he could take an opportunity that would not come round again.
And quite an opportunity at that. Lord Carrington, who had the job from 1984 to 1948, famously decried its lack of power. But Nato was very different then. It knew exactly what its role was. If it had to act, it had by definition failed. Post-Bosnia and Kosovo, this is an organisation that is still evolving to play a wholly new role. Its political command structure evolved greatly during the Balkans war, after an unwieldy start in which the 19 Nato ambassadors were virtually poring over target maps. But a new structure needs to be formalised.
One of the biggest questions in European politics - some would say the biggest - is how Europe can increase its ability to act with less dependence on the Americans, but without stimulating isolationism in the United States. For this Robertson is well-suited, as one of the baby boomer generation of centre-left politicians dominating the Western Alliance, and one who has long been as much a European as he is an Atlanticist. This is important to what would have once been the wholly exotic concept of a British Labour politician running Nato. Robertson was an Atlanticist and a pro-European when it was deeply unfashionable in his party to be either. It neatly closes a circle for him to be doing a job that absolutely requires him to be both.