Alan Watkins: A title is worth shedding for the top job

Peter Mandelson might resign his peerage and return to the Commons – but only if the prize was the premiership

This week it is Lord Mandelson's turn to assume the position of acting prime minister. In reality, Mr Gordon Brown is in constant touch, by various electronic and no doubt other means including telepathic and divine intervention, resting neither by day nor by night, God help us all. In recent years, the silly-season practice has arisen of speculating about who, if anyone, is meant to be in charge of what Lord Beaverbrook once called the clattering trade.

Most recently of all, a sort of duty officers' roster has been published: first Ms Harriet Harman, who has just completed her tour; now Lord Mandelson, then Mr Jack Straw, and Mr Alistair Darling. In quite what order the last two are meant to appear before our admiring eyes, I have now forgotten. In any case it does not matter much.

Labour has had only two deputy prime ministers: John Prescott and Lord Mandelson's maternal grandfather, Herbert Morrison. Morrison's position was not, as far I know, ever formally announced. The new prime minister, Clement Attlee, said grudgingly to Morrison: "As you're there, you might as well do the job yourself, Herbert," or words to that effect. Morrison took over the deputy leadership too; he was not elected by his colleagues until 1952.

Lord Mandelson, who takes a pride in the history both of his family and his party, will know that in 1945 his grandfather might, just, have become prime minister himself. Following Labour's huge victory of that year, an intrigue was mounted to re-elect Attlee as leader of the parliamentary party and, consequently, as prime minister. The conspirators had Morrison in mind. The future foreign secretary, Ernest Bevin, said to Attlee: "You get down to the palace quick, Clem."

Ms Harman is not deputy prime minister. There is no real reason to fill the position at all. With the Conservatives, the post has come and gone with political fashion and the requirements of the moment: Anthony Eden, R A Butler, William Whitelaw, Geoffrey Howe and Michael Heseltine.

In the 1960s and 1970s, George Brown, Edward Short and Michael Foot were all deputy leaders of the party during periods of Labour government, when none of them was made deputy prime minister. The truth is that the job is as unnecessary as that of deputy PM. It was invented for Morrison when he lost his place on the National Executive Committee as the result of a massacre at a party conference.

Even so, my view is that Ms Harman should have been given a full run, unimpeded by possible rivals. Equally, I think that the most likely replacement for Mr Brown – which may surprise you – is Mr Darling, with Mr Straw as the Gromyko figure. Andrei Gromyko, as perhaps I should remind my younger readers, was the Soviet foreign minister in the days of the Cold War, who survived numerous changes of regime while retaining his place in the group photographs. Mr Straw's latest move, no doubt thoroughly justifiable, is to pitch for the south London vote.

But it is of Lord Mandelson that everyone now seems to be writing. All the commentators that I have read recently, have written that someone – Mr Brown, Her Majesty, the queen of the fairies – can somehow wave a magic wand and return the noble lord from the upper house to his old haunts,

interrupted by his service in Brussels, in the House of Commons.

These matters can be arranged: in a moment, in the twinkling of an eye. Before our very eyes, Lord Mandelson would become a mister once again. The appropriate legislation, we are told, is in place. Indeed, some accounts tell us that it already exists.

This is not so – or, if you prefer, not quite so. Hereditary peers can now stand for and sit in the House of Commons. This is more recent than the famous changes of the 1960s, when Tony Benn showed the way by disclaiming his peerage and both Quintin Hogg and Alec Douglas-Home benefited themselves – and perhaps their own party – by disclaiming theirs.

A peer, Lord Thurso, sits for Caithness as John Thurso. He is the grandson of the former leader of the Liberal Party, Sir Archibald Sinclair.

Another former Liberal, who became a Liberal Democrat, Andrew Phillips, was made a life peer in 1998 as Lord Phillips of Sudbury. He was a public-spirited solicitor who possessed numerous admirable interests. After some years, he said he wanted to renounce his peerage. He was told he could not do this. In 2006, he introduced a Bill into the Lords to enable this to happen. The Lords passed the first reading. Since then, as far I can tell, nothing has happened, though there were several messages of encouragement and goodwill. There the matter rests.

But can a prime minister return to the Lords as a prime minister? Could Lord Mandelson perform from the red leather benches rather than from the green of the lower house? The Blair years have already eliminated the hereditary peers. We are now promised a wholly elected upper house. Change is supposed to be in the air. Why should Lord Mandelson not make the first move, or have it made on his behalf?

The last prime minister to sit in the Lords was Lord Salisbury in 1902. Since then it has been laid down by numerous grave authorities that no member of the House of Lords can ever be prime minister. The rule, or the convention, is said to have become established in 1923 when Stanley Baldwin was preferred by King George V over George, Lord Curzon.

One of the people consulted by the King was the former prime minister, Arthur Balfour (there had been four successors to Balfour before the new vacancy occurred in 1923). The general expectation was that Curzon would win. But Balfour returned to the grand house where he was staying to be asked by his equally grand hostess:

"And will dear George be chosen?"

"No," Balfour replied, "dear George will not."

The story immediately became current that he had been eliminated by the King because he had been a member of the Lords. Curzon himself helped to propagate the tale. He raised it specifically with George V. The King agreed it had been a factor in his choice but that it was not the whole story. When Curzon protested that it was his peerage that had done him in, the kin said: "I didn't say that." The fact was that Curzon was intolerable.

In 1940 Lord Halifax, not Winston Churchill, was seriously considered as a successor to Neville Chamberlain. In the post-war period, Lord Home was made foreign secretary three years before he had renounced his peerage to become prime minister. Lord Carrington was foreign secretary later on. Evidently, there is no reason why Lord Mandelson should not succeed Mr David Miliband. The post of Chancellor might present greater difficulties if he were in the Lords.

The sad truth is, however, that all this speculation is a sign of a government that is collapsing by the month. Lord Mandelson's highest hope is that he might become leader of the opposition, which might not suit him at all.