WASHINGTON—Mitt Romney's success in raising hundreds of millions of dollars in the costliest presidential race ever can be traced in part to a secretive data-mining project that sifts through Americans' personal information—including their purchasing history and church attendance—to identify new and likely, wealthy donors, the Associated Press has learned.

For the data-mining project, the Republican candidate has quietly employed since at least June a little-known but successful analytics firm that previously performed marketing work for a colleague tied to Bain & Co., the management-consulting firm that Mr. Romney once led.

The head of Buxton Co. of Fort Worth, Texas, Chief ...

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