UK Prime Minister David Cameron MP says that he fully intends to to fight the next election (in 2015) and serve another full term as Prime Minister – meaning that he will be Prime Minister until 2020. But you can't take what any politician says quite literally.

For a start, Mr Cameron's vision of staying in power until 2020 depends on the Conservatives winning the 2015 election. This is by no means certain. But evenif the Conservatives did win in 2015, what would the future hold? A narrow victory would still leave Mr Cameron in a weak position. There would be plenty of people jostling for his job, and a series of knife-edge parliamentary debate would do nothing to strengthen his hand. A strong victory should, you might think, make his leadership unassailable. But would it?

There would still be doubt. What does he say a year or two before the 2020 election? That he will step down at the end of his term and let someone else fight the election? Changing leaders just before an election is no way for a party to win – particularly if the leadership contest has been divisive. The public need time to get used to party leaders and know what they are voting for.

Nor could he say he would fight the election and then let someone take over. The public won't take kindly to voting for one person then getting another. Could he perhaps say that he will fight the election, serve a couple of years, then let someone else take over then? Once more, the public might well ask why they should elect someone who quits halfway through the job.

Mr Cameron's problem is faced by all Prime Ministers. For the party, the ideal is to go halfway through your term of office, so that your successor can get experience and public familiarity before fighting the subsequent election. But you can't say this. And if your colleagues take you at your word that you intend to carry on too long, they are quite likely to give you the push anyway, as Margaret Thatcher, despite winning three elections, found out.

The only option for any Prime Minister loyal to his or her party is duplicity. You have to tell the public that you will fight for a second term and see it out, but you have to make way for your successor before your time is up. In any case, seven and a half years is plenty for a Prime Minister. Any longer and, no matter how great you are, your accumulated baggage starts to weigh you down.