A case involving AT&T that goes before the U.S. Supreme Court this week has sweeping ramifications for potentially millions of consumers.

If the court rules for the telecom, any business that issues a contract to customers, such as for credit cards, cell phones or cable TV, could prevent them from joining class-action lawsuits.

This would take away one of the most powerful legal tools available to consumers in such cases, particularly those involving relatively small amounts of money. Class-action suits allow plaintiffs to band together in seeking compensation or redress, giving more heft to their claims.