In the weeks before the New Hampshire primary last month, Linda Twombly of Nashua says she was peppered with online ads for Republican Senate hopeful Jim Bender.

It was no accident. An online tracking company called RapLeaf Inc. had correctly identified her as a conservative who is interested in Republican politics, has an interest in the Bible and contributes to political and environmental causes. Mrs. Twombly's profile is part of RapLeaf's rich trove of data, garnered from a variety of sources and which both political parties have tapped.

RapLeaf knows even more about Mrs. Twombly and millions of other Americans: their real names and email addresses.

This makes RapLeaf a rare breed. Rival tracking companies also gather minute detail on individual Americans: They know a tremendous amount about what you do. But most trackers either can't or won't keep the ultimate piece of personal information—your name—in their databases. The industry often cites this layer of anonymity as a reason online tracking shouldn't be considered intrusive.

RapLeaf says it never discloses people's names to clients for online advertising. But possessing real names means RapLeaf can build extraordinarily intimate databases on people by tapping voter-registration files, shopping histories, social-networking activities and real estate records, among other things.

"Holy smokes," says Mrs. Twombly, 67 years old, after The Wall Street Journal decoded the information in RapLeaf's file on her. "It is like a watchdog is watching me, and it is not good..."
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