The deal Paisley must say "No" to

Tom Griffin (London, The Green Ribbon): Jacqui Smith has postponed the second reading of the counter-terrorism bill, The Guardian reported yesterday, in the hope that 9 MPs from the Democratic Unionist Party can be persuaded to back the Government's proposals for 42 day detention:

The DUP, whose leader Ian Paisley will step down in May, is thought to be willing to enter into talks over support for the measure in return for delaying the devolution of policing and criminal justice in Northern Ireland.

This would be a particularly shabby arrangement for both sides. For the British Government, it would mean abandoning its own timetable for the devolution of justice set out in the St Andrews Agreement, while the DUP would be seen to have horsetraded its position on a great public issue, and for what? The party's deputy leader Peter Robinson claimed to support devolution in principle when it was debated in the Commons:

The Minister will know that the DUP is a party that believes in devolution of the fullest possible kind. It thus believes that policing and justice powers should be devolved in the right circumstances and at the right time. He knows that that is our position because it was the position that we advanced to the Government back in 2004 when we considered matters that led to the Government's comprehensive agreement. We made it clear that we were working towards the devolution of policing and justice powers. That is the endeavour of the DUP.

One thing is certain. If DUP votes help put 42 days detention on the statute book, it will still be there long after the party has judged the time is right for Stormont to take over policing and justice powers.

If Robinson is to be believed, there is little reason for such a fateful deal, because his party already has an effective veto over devolution:

The DUP negotiated the triple lock mechanism some years ago, and it is a sensible piece of legislation. It ensures that the First Minister must approve the devolution of powers before the matter gets to the Assembly, that the Assembly must approve the devolution of powers by a majority of both designations, and that Parliament must approve the devolution of powers.

The Government may attempt to come up with some other quid pro quo. In 2005, Ministers offered the DUP concessions over the future of the Royal Irish Regiment in return for a vote in favour of 90 day detention. They spurned that particular deal, helping to inflict Tony Blair's first Commons defeat as Prime Minister.

More ominous precedents are not hard to find, however. There is a long history of Irish parties doing opportunistic side-deals at Westminster. The tactic was perfected in the nineteenth century by Charles Stewart Parnell's Irish Parliamentary Party, which helped to create the modern whip system through its attempts to hold the Commons hostage to the cause of Irish Home Rule.

Since partition and the rise of Sinn Féin abstentionism, this approach has largely been the preserve of unionists. In the 1970s, it was Jim Molyneaux's Ulster Unionists, masterminded by Enoch Powell, who shored up Jim Callaghan's government. In return, Callaghan increased Northern Ireland's representation in the Commons from 12 seats to 17. David Trimble performed a similar service for John Major in the 1990s, in return for the promise of a Northern Ireland Grand Committee. Nationalists have long charged that Major's dependence on the unionists disrupted the peace process until Labour came to power.

An arrangement that invites comparisons with the fag-end of the Callaghan and Major eras is surely not an auspicious prospect for Gordon Brown. There are also dangers for the DUP in being seen to do a side-deal. It would simultaneously demonstrate the party's lack of principle in relation to British civil liberties, and the Government's lack of principle in relation to Northern Ireland. By lining up with the payroll vote against the Tories, Lib Dems and Labour backbenchers, the DUP would be affirming that their priorities are fundamentally removed from those of mainstream British politics. That would be powerful ammunition for those who believe that democracy on both sides of the Irish Sea would be enhanced by breaking the connection between Westminster and Northern Ireland altogether.