Gordon Brown is today engulfed in crisis after a key aide resigned and the Tories threatened legal action over explosive leaked emails discussing how to smear senior Conservatives, including David Cameron's wife, Samantha, with rumours about their private lives. Damian McBride, one of the prime minister's closest advisers, quit over his exchange with the Labour blogger Derek Draper, in which the two discussed setting up a website to air scurrilous allegations about opponents, including unfounded allegations about affairs between leading opposition MPs. The idea was still being actively discussed until a fortnight ago, the Observer has learned. ..... Among the...