When software firm MobilVox wanted to break into the lucrative world of defense contracting, it pursued an unmistakable strategy: It expanded operations from its Northern Virginia base in Rep. James P. Moran's congressional district to the southwestern Pennsylvania district of Rep. John P. Murtha. Working with two of the most powerful members of a House subcommittee that controls Pentagon spending, the company also hired lobbying firms that employed former top aides of both the Democratic lawmakers and Mr. Murtha's brother. Company executives and their lobbyists donated thousands of dollars to the two congressmen. Soon, money flowed the other way. Between...