Wal-Mart Stores Inc. is mobilizing its store managers and department supervisors around the country to warn that if Democrats win power in November, they'll likely change federal law to make it easier for workers to unionize companies -- including Wal-Mart.

In recent weeks, thousands of Wal-Mart store managers and department heads have been summoned to mandatory meetings at which the retailer stresses the downside for workers if stores were to be unionized.

According...