Individuals are the single largest source of campaign contributions, yet we know little about their motivations. For instance, the existing literature questions whether individual contributors sophisticatedly differentiate among candidates according to policy positions, particularly among same-party candidates. We analyze this issue by combining data from a new survey of over 2,800 in- and out-of-state donors associated with the 2012 Senate elections, FEC data on contributors' professions, and legislative records. Three major findings emerge. First, policy agreement between a donor's positions and a senator's roll calls significantly influences the likelihood of giving, even for same-party contributors. Second, there is a significant effect of committee membership corresponding to a donor's occupation; this holds even for donors who claim that other motivations dominate, but it does not appear to be motivated by an expectation of access. Third, conditional upon a donation occurring, its size is determined by factors outside a legislator's control.