Lots of people have come up with various permuatations of this. The best-known recent use was Peter Mandelson after comfortably keeping his Hartlepool seat in the 2001 general election. He said: 'I am a fighter not a quitter...It was said I was facing political oblivion...well, they underestimated me.'
Richard Nixon said somthing similar during the controversy over Watergate - and then went on to resign. As well as saying the memorably erroneous "I am not a crook".