Tom McCabe's contribution to the constitutional debate within the Labour Party at the weekend attracted some pretty dramatic headlines. Understandably so. His arguments were well-made and relevant. They need to be heard and debated, and there can be no more appropriate a time than during a leadership contest.

He is right to identify the method of election as apparently paradoxical. If the new person in charge of Labour at Holyrood is no more than the leader of our MSPs, then why is every section of the party entitled to a vote? Why am I, as an MP, given the same say as an MSP in the appointment of someone who will have no authority over me or the party outside the Labour group of MSPs? The answer is simple - Labour's leader in Holyrood gains authority from the breadth of mandate from all sections of the party. Creating a new post of "Leader of the Scottish Labour Party" would be so fraught with difficulties as to make it entirely impractical. Under such an arrangement, Labour MPs at Westminster would owe allegiance not to the Prime Minister but to the Scottish leader, and would, presumably, be mandated to support policies on reserved matters that were developed, not on a UK basis, but entirely in Scotland.

This may have some superficial attractions to those of a nationalist bent, but it runs entirely at odds with the Scotland Act and to the 1997 White Paper which stated explicitly: "There are many matters which can be more effectively and beneficially handled on a United Kingdom basis."

By framing all policies, and not just those which are devolved to Scotland, at a Scottish party level, we would be undermining the very institution that we, the Labour Party, created in 1999. The White Paper, based substantially on the years of work undertaken by the Scottish Constitutional Convention, explicitly recognised the value to Scotland of having some major policy decisions taken on a UK basis, with Scottish input. The convention and its members were able to see beyond the narrow, petty political philosophy which asserts that the only good policy is Scottish policy. We must never allow ourselves to go down that questionable path.

One possible solution to the leadership problem might be to allow MSPs exclusive rights to elect their leader. But we all want whoever wins our current leadership contest to become not just leader of the opposition, but First Minister in a few years' time. Shouldn't the wider party, therefore, have a say in that crucial decision?

And is Tom really arguing that Labour would have been more successful in Scotland had Henry McLeish, and not Tony Blair, been leader? Or that Donald Dewar lacked authority because he was technically only the leader of the Scottish group of Labour MSPs? In arguing for the new leader to have overall control of the party machine, Tom claims that Labour MPs have to recognise that Holyrood has become the party's main battleground in Scotland. However, we can actually measure empirically the level of voter interest in the respective elections, instead of having to rely on intuition and anecdote.

In the UK General Elections that have taken place since devolution, voters have turned out in significantly greater numbers than for any of the three Holyrood elections. I happen to believe that, over time, Tom will be proved correct, that Scottish voters will in future see Holyrood as more and more relevant to their lives, but that is not currently the case.

Moreover, it's simply inaccurate to suggest that the organisation of the party in Scotland is "owned" by the MPs. The general secretary is answerable to all the party's stakeholders - MPs, MSPs, MEPs, councillors, members, trade unions, socialist societies and other affiliated organisations - through the party's Scottish executive committee. By choosing to make him accountable instead to MSPs alone, we would be sidelining the vast majority of the party, not just MPs.

Where Tom is right, however, is with regard to the importance of Scottish autonomy in devolved matters. No Scottish Executive led by the Labour Party should seek permission from a Labour Government, Labour Prime Minister or Labour Leader of the Opposition to pursue a particular policy. Devolution was delivered by the Labour Party in the UK. It is the Labour Party which has the responsibility to safeguard and nurture it, and to protect it from attempts by the Nationalists to undermine the delicate and unique balance between devolved and reserved matters, a balance endorsed by Scotland in the 1997 referendum.

One key aspect of Tom's critique is, however, definitely at odds with the reality. Relations between Labour's MPs and its MSPs have never been healthier. MPs, still in government at a UK level, are nevertheless in no doubt that their own political fortunes are inextricably linked with those of our Holyrood colleagues, and share their determination to make sure that our current state of opposition is a temporary one.

Tom Harris is MP for Glasgow South and Transport Minister