Candidates around the country are increasingly turning to an effective but controversial technology known as behavioral targeting in order to reach potential voters online. The ads work by tracking Web users in order to identify the issues they're most interested in.

Businesses have used the ads for years, and both presidential campaigns employ them. But, as David Herbert explains in a new NationalJournal.com online exclusive, downballot candidates are increasingly getting in on the act -- even as Congress has recently held hearings into whether the technology intrudes on privacy rights.