Sez who? He's only donated money to Democrats, and admits to voting in a Republican primary exactly once in his life -- for John McCain!
I have to stand corrected after continuing reading this thread. I was going by Clarke's self-statements (so the answer to your question is "sez Clarke"... which I should have known proves nothing :)
I was not aware of his donations nor of his voting records, just that he claimed to be a Republican. I guess I shouldn't be surprised that his Republicanness turned out to be (what now seems like) a phony front designed to insulate him from criticism.
I was wrong. Clarke is, by all evidence, a Democrat and it now seems likely to me that one can add partisanness to the other charges (of incompetence, insubordination, blinkeredness, etc) against him when evaluating his trustworthiness. Thanks for the correction,