A great deal of research focuses on contributions by political action committees (PACs) to candidates, but PACs are also institutional mechanisms for mobilizing contributions by individuals. Restrictions on the ability of PACs sponsored by businesses, trade associations, and labor unions to solicit contributions and the private benefits of contributing imply that these PACs are likely to mobilize donors who do not otherwise contribute to political campaigns. Analysis of itemized contributions to PACs during the 2004 election cycle confirms this. Moreover, the numbers of donors and dollars contributed to sponsored PACs aggregated by congressional district during 1996-2006 are relatively unaffected by electoral competition, presidential cycles, or changes in campaign finance regulations, and the effects of urbanization are less uniform than for nonconnected PACs. PACs sponsored by economic institutions therefore expand the pool of donors beyond the usual suspects.