Going into Tuesday’s crucial Pennsylvania primary, Hillary Clinton’s task looks not just difficult – as it has for weeks – but nigh on impossible. Overturning Barack Obama’s lead in the contest for the Democratic nomination requires, among other things, convincing victories in all the remaining primaries, of which Pennsylvania is the biggest. The polls offer little encouragement. Mrs Clinton’s once-commanding lead in Pennsylvania has narrowed in recent days, and the picture in Indiana and North Carolina is no better.