MUNDELEIN, Ill.—Robert Brownson long believed that his proposed development here, with its 200,000-square-foot Wal-Mart Supercenter, was being held hostage by nearby homeowners.

He had seen them protesting at city hall, and they had filed a lawsuit to stop the project.

What he didn't know was that the locals were getting a lot of help. A grocery chain with nine stores in the area had hired Saint Consulting Group to secretly run the antidevelopment campaign. Saint is a specialist at fighting proposed Wal-Marts, and it uses tactics it describes as "black arts."

As Wal-Mart Stores Inc. has grown into the largest grocery seller in the U.S., similar battles have played out in hundreds of towns like Mundelein. Local activists and union groups have been the public face of much of the resistance. But in scores of cases, large supermarket chains including Supervalu Inc., Safeway Inc. and Ahold NV have retained Saint Consulting to block Wal-Mart, according to hundreds of pages of Saint documents reviewed by The Wall Street Journal and interviews with former employees.

Saint has jokingly called its staff the "Wal-Mart killers." P. Michael Saint, the company's founder, declines to discuss specific clients or campaigns. When read a partial list of the company's supermarket clients, he responds that "if those names are true, I would say I was proud that some of the largest, most sophisticated companies were so pleased with our success and discretion that they hired us over the years."