PM David Cameron (pictured leaving 10 Downing Street) has said he would not bid for a third term . For many of the greatest leaders in our history, one word was simply beyond them. From Gladstone and Churchill to Thatcher and Blair, they could not bring themselves to say goodbye. So by historical standards, David Cameron’s revelation that, if the voters send him back to Downing Street in May, he will step down after five years and allow somebody else to lead the Conservatives into the 2020 election, counts as a genuine bombshell. For some time, this has been predicted in Westminster. But nobody expected him to say so publicly, and certainly nobody thought he would do it on the brink of one of the tightest and most unpredictable election campaigns in history. Not for Mr Cameron the solemn goodbye in an address to the nation. Instead, he announced his intentions in a throwaway interview with a BBC journalist. It was absolutely typical of the man, the quintessence of laid-back insouciance. Even his rhetoric seemed remarkably casual. Terms in office, he remarked, ‘are like Shredded Wheat: two are wonderful but three might just be too many’. What makes Mr Cameron’s announcement so extraordinary is that there is simply no historical precedent. The great majority of his predecessors tried to cling on to power as long as possible, and had to be dragged kicking and screaming from office. Margaret Thatcher, for example, famously led her party into a third general election in 1987 with a promise that she would go ‘on and on’ – a remark that delighted her admirers and horrified her critics. Few people doubt that had she not been toppled in a coup in 1990, she would have led the Tories into a fourth election two years later. Indeed, if the Iron Lady had had her way, she would have led them into the 21st century, too. But Mrs Thatcher was far from unusual. The Victorian titan William Gladstone led the Liberals to victory in 1892 at the staggering age of 82, long after most of his colleagues thought he should have stepped down. Winston Churchill, too, had to be prised from office like an oyster from a shell. He should have resigned in glory after winning World War II and losing the 1945 general election. Instead, he refused to budge, even though he was increasingly debilitated by old age and a series of strokes. The most obvious recent example, though, is Tony Blair, whose premiership was undermined almost from the very beginning by gossip about the succession. According to Gordon Brown and his friends, Mr Blair had promised to step aside after two elections for his ambitious chancellor. But when he refused to do so, their relations degenerated into virtual civil war. In 2004, Mr Blair promised that if he won a third election he would soon resign to give his successor time to prepare for Labour’s tilt at a fourth term in office. The Brownites were furious at the delay. But far from lancing the boil of speculation, Mr Blair’s announcement only made matters worse. From that moment, his aura of leadership began to seep away. Since everyone knew he was going, they had nothing to lose in challenging his authority. And so it was that Mr Blair’s final two years in office, overshadowed by constant bickering over the date of his departure, were effectively a drawn-out public execution, as the former master of British politics suffered an agonising death by a thousand cuts. I am sorry to say this is almost certainly the scenario that awaits Mr Cameron. And his decision is all the more mystifying because he is under relatively little pressure to step aside. Whatever you think of Mr Cameron, the unarguable fact is that he remains the Tories’ single biggest asset. For the past ten years he has been comfortably more popular than his party. Scroll down for videos . David Cameron has revealed he will stand down as Prime Minister before 2020 if he wins the next election . At the age of just 48, Mr Cameron is, by prime ministerial standards, a relatively young man. On top of that, Mr Cameron is clearly the most popular and credible of the major party leaders. Neither the laughable Ed Miliband or the anodyne Nick Clegg comes close to challenging his public standing. While he has his critics on the Tory backbenches, a leadership challenge would be almost unthinkable if he managed to win another term in May. If I had been advising Mr Cameron, I would have urged him to emulate Harold Wilson, who showed no sign of going anywhere until his shock resignation in 1976. Like Mr Cameron, the Labour prime minister was the dominant political personality of his age. He shrugged off accusations that he lacked substance to win election after election. He even managed to bounce back from defeat in 1970 – as I believe Mr Cameron could, too, if he really wanted to. So Wilson’s resignation, at the age of 60, stunned the nation. There had been not the slightest public suspicion that he was planning a quick exit. By contrast, Mr Cameron has, I think, made a rod for his own back. If he returns to No 10 in May, which looks increasingly likely, then it will be as a lame-duck Prime Minister. For the awkward squad on the Tory backbenches, the penalties for rebellion will seem trivial and short-lived, since a new leader will be along in a year or two. And for the Prime Minister’s ambitious colleagues, the leadership campaign will begin on the day he walks back into No 10. David Cameron and his wife Samantha Cameron invited the cameras into their Oxfordshire home ahead of the election, during which the Prime Minister made the revelation that he would step down before a third term . Indeed, for George Osborne, Boris Johnson and Theresa May, that campaign may well have already begun. And if Mr Cameron sticks to his promise to serve a full five-year term, it will drag on until the beginning of 2020. I can hardly think of anything more likely to undermine and debilitate a future Tory government – especially one with a narrow or non-existent majority. And I firmly believe that at some stage in the next few years – and probably sooner rather than later – Mr Cameron will have cause to regret his decision. It is, of course, to his credit that, far from clinging to the baubles of office, he is already preparing for a dignified exit. But, only weeks before the most uncertain election in living memory, it was surely a mistake to telegraph his intentions as he did. And if Mr Cameron manages to retain power on May 7, then I believe he will bitterly regret the casual manner in which he threw away so much of his authority.
David Cameron said he would not bid for a third term if he wins election . The PM said two terms were 'wonderful' but three 'might just be too many'
If re-elected in May, he will serve a full five years but then quit Number 10 .