I pose the question because the suggestion has been raised by former Prime Minister John Major in a forthcoming interview for Radio 4.   According to The Guardian Mr Major will say:

"I think we have seen examples of politicians carrying on too long.  I personally believe – although I’m not quite sure how you enshrine it – that the American system of somebody serving only eight years is very sound.  If you look historically at prime ministers who have served beyond eight years, it has usually been downhill once the eight-year mark has been passed.  So I think as a self-denying ordinance, leaving after eight years is a very smart piece of politics."

Mr Major may be right that most Prime Ministers and Presidents would be best advised to go after eight or so years but, as he hints, it is difficult to legislate for term limits.  It is easy for politicians to become lame ducks when everyone knows that they are ‘on the way out’.  Tony Blair has lost much of his authority because the whole of his government is now looking to his successor for future direction and patronage.  Surely it is for voters to decide whether a political leader has run out of steam or not?