Abortion is one of the leading issues in the US, sure to animate any discussion of politics and life. For decades now, it has been a key issue for voters and one where partisan lines are predictably drawn between Democrat and Republican parties and candidates. But how has that happened? Where does abortion policy currently sit - and what impact might all of this have on this November's elections, when both candidates have made it a centrepiece of their campaigns - albeit for vastly different reasons.