Concerning donations to political parties, on the state and national level:

Why do individual people donate?  Is there an expectation of increased
access to the political party and/or rewards?  Is the primary
motivation to simply to help the party win in an election, regardless
of personal rewards?

Ideally, I would like to know the percentage of people who currently
donate to political parties but would not donate if not for the
expectation of personal rewards.  This is probably hard to determine,
so answers helping me estimate or understand this would be great.

Does this answer change for donations to political parties from organizations?

Please substantiate your answer with references.  A large survey or
study is acceptable.

Thanks!