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.

The Wal-Mart human-resources managers who run the meetings don't specifically tell attendees how to vote in November's election, but make it clear that voting for Democratic presidential hopeful Sen. Barack Obama would be tantamount to inviting unions in, according to Wal-Mart employees who attended gatherings in Maryland, Missouri and other states.

"The meeting leader said, 'I am not telling you how to vote, but if the Democrats win, this bill will pass and you won't have a vote on whether you want a union,'" said a Wal-Mart customer-service supervisor from Missouri. "I am not a stupid person. They were telling me how to vote," she said.

"This is a David-and-Goliath confrontation, but we believe we'll have enough stones in the sling to knock this out," said Mr. Steven Law, chief legal officer of the U.S. Chamber of Commerce.