John Rentoul: Cameron, the incredible shrinking man

Tory plans to take power away from Westminster invite voters to elect a government that will do less and less for them

If David Cameron wins the election next year, he will be the weakest prime minister in our democratic history. More than any of his predecessors, he is a self-neutering politician. He has promised to give away unprecedented powers if he gets into No 10. This is a process that has been going on, slowly, since the Second World War; but Cameron would take it much, much farther.

It started with Clement Attlee, who signed up in 1950 to the European Convention on Human Rights, which subjected the British government to the power of an international court. The government's power was later circumscribed more effectively when the convention was made enforceable in British courts by the Human Rights Act 1998. Ted Heath limited his successors' scope for action more substantially by signing up to Europe in 1973. Even Margaret Thatcher gave away the right to do anything that interfered with the European single market. Then Tony Blair, despite his latter reputation as a sub-Thatcherite elective dictator, gave up the right to set interest rates, and devolved power to Scotland, Wales, Northern Ireland and London.

Now Cameron proposes the biggest giveaway of all. He wants to give up the power to decide the totals for public spending and taxation. He intends to set up an Office of Budget Responsibility that will decide what gap between spending and revenue is sustainable. The idea is that it would be an independent body to enforce a tougher version of Brown's now discredited golden rule, requiring the Government to balance its budget over the economic cycle. To the innocent observer, George Osborne's criticism of Brown for failing to "fix the roof while the sun was shining" was a cute soundbite. To Osborne, however, it will have the force of law. In America it would probably be called the Fix the Roof Act. Osborne intends to legislate to force himself to pay back the national debt when the economy is growing strongly, and borrow only when growth is below trend. It will be left to him to decide whether taxes and spending are higher or lower, but the gap between the two totals will be fixed by a quango.

That is the strictest self-denying ordinance since Cromwell's day – mind you, the original "self-denying ordinance" of 1645, which banned members of either House of Parliament from holding command in the Army, was notable for Cromwell himself being exempted.

That is not all, though. Cameron is also committed to the Bank of England model for the National Health Service, promising to grant "operational independence" to an appointed board. In practice, this pledge is likely to go the same way as Labour's promise in 1997 to abolish the internal market in the NHS, only to reinvent that particular wheel two years later. But let us stick for a moment to what the Conservatives say they intend to do. Decisions about policing will be handed over to directly elected commissioners. On top of that, Cameron says: "We want nothing less than radical decentralisation, to reach every corner of the country." This aspiration was set out early this year in a policy document with the achingly modern title "Control Shift". The document was founded on the mythical premise that Britain is a more centralised state than ever, in which top-down bureaucrats micro-manage the lives of citizens. It was light on specifics, but the theme of decentralisation is a strong one that runs through all Cameron's speeches (usually in direct contradiction to centralised plans to employ 3,000 health visitors or to put panic buttons on Facebook), and we can be sure that local council leaders will claim a king's ransom from a Conservative government for all his hostages to fortune.

Of course, these are all powers that can be taken back. No parliament can bind its successors, although, in practice, the development of the British constitution is what is known in the academic jargon as path-dependent. Each decision constrains future decisions. Power, once handed over, tends to be difficult to recover. Britain could leave the EU; politicians could overrule the Bank of England; Scottish devolution could be revoked. But to set out the possibilities is to conjure up the obstacles. The Office of Budget Responsibility is like the nuclear bomb: once it has been invented, it would be nigh impossible for a mere politician to say: "Thank you and goodnight; from now on, I'll decide what is fiscally responsible."

Each self-curtailment of ministerial power has a logic of its own. Brown's record of fiscal irresponsibility makes the case that we should legislate for prudence. An independent board to run the NHS may be a silly idea, but you can see why it makes superficial sense for a Tory party distrusted on the issue of socialised medicine. There is an impeccable democratic argument for elected police chiefs, which is only strengthened by the vested-interest opposition of the Association of Chief Police Officers.

But do we really want the cumulative effect of all these limits on parliamentary sovereignty? Should we really obsess so much about the actions of ministers, playing out their micro-dramas on their square inch of political discretion, while real power is wielded by largely unknown and barely accountable appointees to independent bodies?

We are promised a policy blitz from the Conservative Party in the new year: expect it to answer none of these questions. Expect Cameron to continue to contradict himself, promising both to give power away and to use what power remains to give us what we say we want. He may even promise a quango to oversee the abolition of quangos.

"The more he talks, the less he says," Gordon Brown unexpectedly alleged in the House of Commons the other day. It seemed a strange thing to say, yet it made a kind of sense. The more policy Cameron sets out, the less power he would have.

John Rentoul blogs at www.independent.co.uk/eagleeye