The curse of the deputy PM
British deputy prime ministers who go on to be successful in the top job are the exception rather than the rule, so what lessons can the current incumbent draw from history?
The meteoric rise of Nick Clegg has put the focus on the unusual job of the number two, the deputy leader or, in his case, the deputy prime minister.
In the United States, the vice president is only a heartbeat away from the White House. He quite often makes it to the top and quite often also proves successful.
It is different over here, the number two hardly ever reaches the top slot and when it does happen, it is often a disaster.
Why should that be so? It is almost as if the job is cursed.
Roy (now Lord) Hattersley was Neil Kinnock's deputy as leader of the Labour Party for nine years.
He recalls that Al Gore once remarked to him that being vice president of the United States "was not worth a pitcher of warm spittle".
He had replied that being deputy leader of the Labour Party was worse than that.
The curse was that so many incumbents were not really deputy leaders, they were failed leaders.
The deputy leadership became a sort of consolation prize.
Sir Anthony Eden was Churchill's deputy for years, before eventually succeeding him as prime minister.
Eden won an election but then everything went wrong. Eden did not last long.
The veteran political commentator Anthony Howard thinks there is much in the theory that if you wait too long for office, becoming increasingly anxious and nervous as time goes by, it does something to your character.
Michael Foot was widely liked and respected as Jim Callaghan's deputy, but he failed too when he became party leader. Labour was bitterly divided and was in the wilderness for years.
The great exception was Clement Attlee.
He was Churchill's deputy during the Second World War in the last coalition government before the present one.
Then he won a landslide election victory in 1945.
Historian Professor Peter Hennessy says Attlee was a remarkable man whose natural reticence was misleading: "He had hardly any presence.
"He struck people as a kind of benign gerbil and, as Douglas Jay once said of him, 'He'd never use one syllable when none would do'.
"And on the Richter Scale for charisma, the needle didn't flicker really."
But he was an effective deputy to Churchill and one of the greatest peacetime prime ministers.
Margaret Beckett was John Smith's deputy as Labour leader from 1992-1994.
She was the first woman to hold the post and subsequently became the first woman to lead the Labour Party after Smith's death (just as Harriet Harman has stood in following Gordon Brown's resignation).
She says she had had no intention of standing as deputy leader: "I thought it was a terrible job, but the BBC for some reason had a trailer where it said I was planning to run as deputy as a leadership team with John."
She called the BBC to say she was not running but then her phone started to ring and the messages said "Why not?"
Lord Hattersley believes it is very difficult to be a good deputy leader.
"I think I was a decent deputy leader because I was a very inactive deputy leader. I never behaved in a way which was a challenge to Neil Kinnock. I never confronted him over policies, over issues, over performance."
Michael (now Lord) Heseltine, who was John (now Sir) Major's deputy prime minister, does not think the job is cursed: "Certainly not. It was a very exciting job to have and I learned a lot. But it's not the real thing. Of course I would have preferred to be prime minister, but that's not the way the cards fell."
Advice to Clegg
Lord Heseltine thinks no comparison should be drawn between his own position as deputy PM and that of Nick Clegg: "There are bound to be divisions between him and David Cameron, there are bound to be policy issues, there are bound to be personality issues.
"It can certainly be made to work as long as we are mature about it, but any nuance, any gesture will be part of the crumbling coalition, as the media will want to project it."
Lord Hattersley does not think much of Nick Clegg's chances: "If you're asking me about the coalition, it can be one of two things, a facade or a disaster."
Peter Hennessy thinks Mr Clegg could learn from Clement Attlee: "Attlee used to say that cabinet government depends on discussion but it only works if you can stop people talking.
"If Nick Clegg wants to model himself on Clem Attlee, as the one deputy who made a very good prime minister, he should grow a moustache and shut up."
The Curse Of The Number Two will be broadcast on Wednesday 21 July at 2045 BST on BBC Radio 4. It will be available for 7 days after that on the BBC iPlayer