"After all, what have Republican presidents ever done for him?"

Surely the implication of that sentence is not just that Powell should have felt grateful to previous presidents for advancing him (in itself very questionable), but also that he ought to have regarded such personal feelings of gratitude as more important than doing what he believed to be in the good of his country. Is that really what you think Doug? Even if you think he was mistaken in his judgement.
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So far as the imagined notion of Clinton (either of them) backing McCain goes, I think a widespread assumption, if that had happened, would have been that this reflected feelings of resentment, and that it therefore deserved to be scorned. I think many Republicans, even if they felt pleased at the development, would have shared this assumption and felt that scorn.

If it had been Carter on the other hand it would probably have been widely seen, even by many people who felt it was totally wrong-headed, as an honourable decision reflecting a genuine view by an honourable man that McCain deserved to be supported. (I think it might well have made a lot of Republicans a bit more reluctant to vote for McCain, but that is another matter.)