I would guess however that they cannot directly advocate legislation to simply benefit their donors,
They can't provide services in return for contributions, but they can still advocate for legislation that will benefit their donors. To prove bribery, a prosecution has to prove three things:
- That the money was received.
- That the service was provided.
- That the reason that the service was provided was that money had been received as payment for providing that service.
The first two are generally much easier to prove than the third. The "quid pro quo" standard can be difficult to prove beyond a reasonable doubt. Note that an obvious defense argument would be that the legislator would have voted the same way without the money.
Example source: http://www.huffingtonpost.com/bennett-l-gershman/bribery-cases-_b_1590284.html