In the end, it usually comes down to the question of legacy. Prime Ministers almost always end up being fixated on the verdict of history and how they will be remembered decades after we all are gone and their portrait is still on the wall of the staircase in Downing Street.No Prime Minister wants to go down history as a blip, an in-betweener drummed from office. Matthew Parris in The Times earlier this year wrote that just by getting to this year in one piece and still being in office meant that Theresa May will be recorded as Prime Minister 2016-2019. It was a good observation. That last number being a nine rather than an eight shouldn’t make a difference to anything, really, but somehow it does. Three years looks serious. Two years would have looked like a blip.Similarily, at the time and afterwards in the press, there was much speculation on why Jim Callaghan (one of the great patriots of the proper Labour movement) did not take his chance in the Autumn of 1978. He might have won that year and Margaret Thatcher would not have got to Number 10. Yet he delayed for eight months, hoping for an improvement in his chances and lost. But waiting meant he was Jim Callaghan, PM, 1976-1979, rather than 1976-1978. To a Prime Minister, it is better.