They did it before. Unionists and nationalists once before united in an attempt to make a fresh start in Northern Ireland, in the Sunningdale agreement of 1973-74, in a deal eerily similar in many respects to that which is now on the table. It was hailed as a new dawn; but in the face of concerted Protestant wrath and a loyalist general strike, it lasted less than five months.
The Point of No Return, Robert Fisk's book on that extraordinary episode, was aptly sub-titled: "the strike which broke the British in Ulster". It was highly accurate in that it was over a decade before a British government plucked up the courage to take a fresh initiative in the face of the discouragement felt after the strike. There have been many changes since then, yet a surprising amount of the political grammar remains intact.
What the British and Irish governments are now engaged in has been called "Sunningdale Mark 2", and even "Sunningdale for slow learners". Success in the enterprise will depend on the hope that many people have learned many lessons since that first time around.
Sunningdale was a political initiative on an epic scale. For 50 years, Northern Ireland had been controlled by the Unionist party, which enjoyed an unbroken run of one-party rule at Stormont. Then Edward Heath swept it away in 1972, concluding after Bloody Sunday that it was unreformable. Heath and the first Northern Ireland Secretary, William Whitelaw, then spent most of 1972 and most of 1973 trying to put together a scheme that might be characterised as Stormont with Catholics. It came in two installments.
The first, in late 1973, was the announcement that Unionist leader Brian Faulkner and the SDLP had agreed to form a coalition to run a new assembly. The executive was composed of six Unionists, four SDLP ministers, and one from the middle-of-the-road Alliance party. That was the powersharing executive; next came what was known as "the Irish dimension." This was hammered out over several days and nights at a conference centre at Sunningdale, in Berkshire, with Heath pushing for agreement in late-night sessions much like Tony Blair's efforts in Belfast this week.
The result, known as the Sunningdale agreement, provided for a new Belfast- Dublin Council of Ireland with functions similar to that of the cross- border body now on the table. When Faulkner wrote in his memoirs that "the problem seemed to be the executive and harmonising functions of the Council of Ministers", he could have been describing the issue that proved so contentious this week.
The new executive came into office in January 1974, with Faulkner as chief executive and John Hume as minister for commerce. In those days, Sinn Fein did not exist as a political force, but the IRA perfunctorily dismissed the whole enterprise as an attempt to "prop up the British presence". The republicans have come a long way since those days.
But before the IRA could launch an organised assault on the deal, loyalists beat them to the punch. First, the Rev Ian Paisley and his political allies disrupted the assembly, one official recalling: "Faulkner was spat upon, jostled, reviled and shouted down. It was sad to see him spat upon by lesser men, political pygmies and procedural bullies and wild men of the woods and the bogs."
Next, the February 1974 British general election produced a disastrous result, anti-powersharing Unionists winning 11 of Northern Ireland's 12 Westminster seats. Then came the loyalist strike, which was run by a committee including Unionist politicians, loyalist paramilitaries and Protestant trade unionists.
An uncertain response from the Labour government of the day gave the strikers almost complete control over electricity generation, allowing them to run down power supplies and cause frequent power cuts. A state of emergency was declared. Workers were turned back by men carrying clubs and blocking roads with hijacked vehicles: relatively little overt force was used on the streets, the paramilitary leaders finding that what one of them drily called "intimidation without violence" was sufficient deterrent to send the workers back home.
But within days Protestant opinion swung behind the strike, and after a few weeks it became apparent that the powersharing executive, and indeed the government itself, had lost almost all authority. The Sunningdale agreement collapsed; with it went most of the hope for an agreed settlement; and many politically arid years followed.
That was then: this is now, and many things have changed. Protestant industrial muscle is not what it was, some of those who supported the strike - like David Trimble - now believe more in the political way than they used to, and the leaders of the major loyalist paramilitary groups say they want to talk rather than fight.
Republicans are signalling that they want to move away from violence and into politics: not everyone believes them, but the idea of a peace process has permeated the atmosphere, implanting the notion in some unlikely quarters that there may be a better way. But some things have not changed.
Mr Paisley, then as now, is gearing up for a major campaign in the politics of denunciation and the rhetoric of sell-out. While it is tempting for outsiders to dismiss him as a melodramatic anachronism, the fact is that he retains a potent appeal, two years ago taking 36 per cent of the Unionist vote. His tactic will be to create splits in the ranks of other Unionist and loyalist groups. There are already unhappy voices in Mr Trimble's own party, four of his 10 MPs being dead set against any deal: Mr Paisley will be intent on fanning such embers of dissent. On the paramilitary front the Loyalist Volunteer Force, a fierce new outfit, will use the gun against a new agreement. They and the other deal opponents will hope to exploit the July Drumcree marching controversy to rally support.
The realignment of forces that has taken place since 1974 means there is less chance of the loyalist ultras succeeding in bringing down a deal. The fact remains, however, that they intend to try, and in doing so will provide a new deal with its sternest test.