imperfect societies, it's much more blurred.

Here's where the blur occurs: In one case, the official receives money for the exchanged services, in the other, he gets elected (with its power and perks) for the exchanged services. In both cases, the official has gained something for some form of exchanged service.

If the "elected" official had just said "thank you for your support of my election" and left it at that--then it's a completely above the board. However, when those donations are done based upon the expectation that the elected official will then use his/her position to benefit the donor, then we have a problem.