In an insightful interview with CounterPunch.org, Drutman points out the lack of direct effects of campaign contributions (which are different from lobbying, generally) on voting behavior. Consistent with some other studies on corporate political activity, the story seems to be more about access and possible influence, rather than direct purchase of legislative benefits:
There is no one-to-one relationship between campaign contributions and favorable outcomes. There is no one-to-one relationship between campaign expenditures and electoral outcomes. But, money is not irrelevant. On the money-buys-outcomes story, campaign contributions help access, and access is necessary for influence. But it’s possible to have access without making contributions, and plenty of actors have access but no influence.
Or perhaps more simply: by making a contribution, you are buying a ticket to the main event. But there are far more tickets for sale than there are prizes.
Definitely worth reading.