This is not the memory I have of the saying - I think it was a US comedian where I first heard it but googling brings this up:

"Broadcast on The Westminster Hour, Sunday March 31 2002

One of the first lessons a young politician learns is that while the other parties may be your opponents, you should always look among your own side to find your true enemies. And relations with hostile colleagues can often be a defining force in a political career. What's more, these political rivalries are more than personal tales - they have extensive political consequences. In this series I will be exploring some of the most intriguing political rivalries of the twentieth century. Let's start with the long-running feud between two great figures in Labour party history, Ernest Bevin and Herbert Morrison, which provides us with our title. The story goes that someone once remarked of Morrison that he was his own worst enemy, and Bevin immediately butted in to say "Not while I'm alive, he ain't". Perhaps it's too good to be true, but without doubt it accurately represents Bevin's hatred towards his Labour colleague."


http://news.bbc.co.uk/2/hi/programmes/the_westminster_hour/1899102.stm