One of the most prolific donors to political candidates running for federal office has no idea that’s his status.

Guy Bowers, a 66-year-old ex-detective who says he owes his fortune to an inheritance and some savvy investing, is not your typical corporate executive often associated with fattening politicians’ campaign accounts.

Yet Bowers was such an enthusiastic donor that he tops a list of perhaps hundreds who appear to have broken a campaign finance law that caps the total amount of money individuals can give federal political candidates and committees in the course of a two-year election cycle. For 2011 ...