In an interesting article Bruce Newman made the distinction between Teflon and Velcro candidates, to the Teflon candidates nothing bad sticks (think here of Ronald Reagan the so-called Teflon President); alternatively the Velcro candidate is unable to shake off anything negative. Sure its perceptual, and it often relates to the perceived character of the individual, but it is said to be a definer in American presidential races.
Re-reading this made me think that while the contrast is interesting within a campaign, the transition from one to the other may also occur during a political career. Maybe the apocryphal saying that 'All political careers end in failure is correct', consider that of Tony Blair. In the 1997-2000 period nothing stuck to him. Mandelson was demonised for a variety of reasons, often he took the blame for spin despite the fact that Blair employed him as Minister without Portfolio to be de facto Minister for Spin. But Blair was treated as untouchable, the Sun article that accompanies the front page (right) sees Mandelson as the evil within Labour, with his departure all would be well. Then the tide turned, since 2001 incrementally Blair has become more and more like Velcro; every bit of detritus associated with his period of office has become attached to him independent of whether it was directly his responsibility or not. Hence his persona shifted, from blameless to blame-full. This seems set to continue until 27th June, especially perhaps if Brown has his way as he does not want to be too much like Velcro himself, and perhaps it is the negatives that will remain as his legacy, unfair or not. The reason is that he is perceived negatively, therefore it is easy to attach further negatives to him, humans like to balance their beliefs and attitudes!