Boris Johnson ... 'Never underestimate the tendency in British politics to overestimate the importance of personalities.' Photograph: Frank Baron for the Guardian A spectre is haunting British politics -- the spectre of Boris Johnson. David Cameron's faltering leadership of the Conservatives is dogged by whispers that party donors and backbenchers are lining up London's mayor as his replacement. Liberal columnists tug at our sleeves, urging that we take seriously the prospect of a Johnson premiership. Senior figures within the Labour party are said to view him as a genuine threat . When I mention "the coming of Boris" to leftwing friends, haunted looks pass over their faces as they hurry their children indoors, murmuring gravely about the dangers of underestimating him. Their fears are not allayed by last week's poll showing that the bankers' cheerleader with a history of casual racism is now the most trusted politician in the country. Never underestimate the tendency in British politics to overestimate the importance of personalities . It would be foolish to predict that Johnson will not win the 2015 election if he does take over from Cameron. But it's long past the time to set out the reasons why it will be very difficult indeed for him or any other leader to pull this off. The Conservative party's prospects have become increasingly bleak, irrespective of the leadership it chooses or the image it projects. It has not won a general election for 20 years. It has not won one comfortably for 25