Nearly 40 years ago, when the Conservatives were in power and Harold Macmillan was prime minister, there occurred one of several great Tory defence disasters. On this occasion it was the cancellation of the Blue Streak missile. The cartoonist Vicky showed Macmillan, got up as Supermac, saying: "Ha, ha! This puts old Gaitskell in an awful dilemma."

And so it did. As so often with Vicky, what had been intended as a joke turned out to be no more than flat political truth. For two years Labour tore themselves to bits over missiles which they did not understand - for to most people these systems were merely words on paper - and which, in any case, they had no power to put into operation because they were in opposition.

These disputes were to recur in the early 1970s and in the 1980s. They always happened when the party was in opposition, never when it was in government. As prime ministers, Harold Wilson and James Callaghan were allowed to get away with most things in matters of defence - much as Mr Tony Blair today is allowed to get away with even more everywhere.

The Conservatives are now behaving over the single European currency almost exactly as Labour used to behave over defence. There is, however, one difference. When they had one, they did not give their Prime Minister a rest. They made Mr John Major's life a misery.

He brought much of the unhappiness on himself, by devoting his first years of office to the parliamentary approval of the Maastricht treaty, with numerous thrills and spills along the way. True, under the European Parliament Elections Act 1978, the treaty, as a European treaty, needed parliamentary approval before ratification. It could not lawfully have been ratified solely under the prerogative powers of the Crown. Mr Major's spokesmen were mendacious to say it could be. Still, he did not have to take so long about getting the Bill through the House.

Well, the caravan has moved on, but the dogs are barking as vigorously as ever. In 1990-97 they mattered because they could have brought down the prime minister, the government or both. Over the treaty they nearly did. And two years ago the then prime minister chose to submit himself to re-election by his own party - an unprecedented course whose constitutional propriety was questioned only by Mr Enoch Powell and myself. When Mr Major managed to secure the support of two-thirds of the party, Lord Cranborne and his hired hands rushed to the microphones to proclaim a famous victory.

The noble lord in question, a nice enough chap, has now transferred his loyalty to Mr William Hague. But it no longer greatly matters what he thinks, what Mr Hague believes, what Mr Michael Heseltine gets up to or what Mr Kenneth Clarke's latest wheeze may be. The last two are not even on the front bench. They are more experienced than those who are. They are clearly more fun as politicians. But as serious figures they are now, as Lord Curzon said of Stanley Baldwin when the latter was preferred to him as prime minister, persons of the utmost insignificance.

Certainly they have the ability to write newspaper articles or, more likely, to have them written on their behalf, and the consolation of seeing those articles in print. And they have the capacity to appear on Today, The World at One and Newsnight, with the flattering likelihood of seeing their comments recycled in next day's papers. But of political power they possess nothing, any more than Mr Hague, Lord Cranborne and the rest do.

If Mr Blair had a small majority or none at all, or if this parliament were approaching the end of its life, the position might be different. But Mr Blair has an overall majority of 177 and over four years to go as prime minister. What the Conservatives think about a single currency is not worth a packet of jelly babies or, as Madam Speaker Boothroyd (my candidate to be the first President of the Republic) might prefer, a tin of Zubes.

This is not power worship on my part. On the contrary: it is precisely because I believe that the exercise of power should be rigorously scrutinised and closely controlled that I regard the concentration on the misfortunes of the Conservative Party as unhealthy. Irrespective of whether we are observing the Tories with grief or delight, Mr Blair and Mr Gordon Brown are manifestly laughing all the way to Brussels and back. They remind me rather of the two little dickie-birds who were sitting on the wall. They were called, not Tony and Gordon, but Peter and Paul: "Fly away Peter, fly away Paul./Come back Peter, come back Paul."

There is also a resemblance between Mr Brown and the Grand Old Duke of York who, it will be remembered, had ten thousand men. He marched them up to the top of the hill and he marched them down again: "And when they were up they were up,/And when they were down they were down,/ And when they were only half way up they were neither up nor down." Not since the great days of Wilson as prime minister in 1964-67 had a I heard quite such a slippery statement as Mr Brown delivered on Monday. Harold at the height of his powers could not have been more evasive, and poor Mr Peter Lilley proved as ineffective as Lord Home and, later, Sir Edward Heath used to be.

Oddly enough - this is an indication of how Mr Brown managed to muddy the waters - all the briefings turned out to be accurate, all the leaks true. Mr Charlie Whelan has no need to enter a monastic order or even to take a seat at the back of the kirk. The Financial Times story was true to the extent that a government has said for the first time that there is no objection in principle to our entering the single currency. But the Times story was true likewise because Mr Brown said as well that it was "not realistic" to think we could enter within the lifetime of the present parliament.

At the same time Mr Brown urged - no, commanded - our captains of industry to make the crooked straight and the rough places plain in preparation for our triumphant entry into the single currency in ... well, when, precisely? Are the preparations really going to take five whole years, when we managed the 1971 decimalisation in a shorter time? There is no point in fiddling about with your cash registers or your computers unless you know you are fiddling to some purpose.

Mr Brown then proceeded to lay down five criteria. Experience has taught me to be suspicious of politicians who set down conditions which have to be met before something happens. People forget what they are; the criteria mysteriously expand or contract; it is invariably a matter of opinion whether they have been satisfied. Mr Brown's are: (i) whether there can be sustainable convergence between Britain and the economies of a single currency; (ii) whether there is sufficient flexibility to cope with economic change; (iii) the effect on investment; (iv) the impact on our financial services generally; and (v) whether it is good for employment. Make of that what you will. It can mean anything or nothing. Quite apart from Mr Brown's - or the Treasury's - inelegant phrasing, it is the higher gobbledegook which leaves the Government a completely free hand.