I don't know much about politicians, really, but I don't find it difficult to imagine that many of them could believe they are doing neither good nor harm on any significant scale--perhaps that they could not do significant good or harm--, but by means of some combination of seeking benefit for their allies and doing what they feel is necessary to be politically successful are in fact doing more harm than they imagine, at least by comparison with a hypothetical replacement of greater intelligence/integrity/courage/rationality/etc. I'm just speculating, though, and I'm not sure what could be offered in response except anecdotal (and indirect) evidence.