Source http://www.opensecrets.org/pres08/contrib.php?id=N00000286&cycl...

This table lists the top donors to this candidate in the 2008 election cycle. The organizations themselves did not donate , rather the money came from the organization's PAC, its individual members or employees or owners, and those individuals' immediate families. Organization totals include subsidiaries and affiliates.

Because of contribution limits, organizations that bundle together many individual contributions are often among the top donors to presidential candidates. These contributions can come from the organization's members or employees (and their families). The organization may support one candidate, or hedge its bets by supporting multiple candidates. Groups with national networks of donors - like EMILY's List and Club for Growth - make for particularly big bundlers.