Retail giant Wal-Mart on Wednesday asked the US Supreme Court to overturn a lower court ruling allowing more than 1.5 million women employees of the company to join together in what would become the largest class-action employment lawsuit in history.

The lawsuit filed by six women in 2001 charges that Wal-Mart engaged in gender discrimination by paying female employees less than men, and in passing women over for promotions that went to men. It seeks billions of dollars in damages.

Gender discrimination lawsuits are usually litigated one employee at a time. But courts may allow plaintiffs who were harmed under similar circumstances by the same person or company to join together in a common class of litigants to pursue their lawsuit.

Lawyers for Wal-Mart are fighting court rulings in California upholding the creation of the massive class of plaintiffs. It includes all women who worked in any of the company’s 3,400 domestic stores since 1998.

The most recent ruling came in April when the full Ninth US Circuit Court of Appeals in San Francisco voted 6 to 5 to uphold the class action status of the lawsuit. Judges upholding the suit said the large number of affected women didn’t matter and that the suit would be manageable.

Dissenting judges said the large size of the class suggests that the women may not share a common injury. “They have little in common but their sex and this lawsuit,” said Chief Judge Alex Kosinski in a dissent.

The chief judge added that the class members “held a multitude of jobs, at different levels of Wal-Mart’s hierarchy, for variable lengths of time, in 3,400 stores, sprinkled across 50 states, with a kaleidoscope of supervisors (male and female), subject to a variety of regional policies that all differed depending on each class member’s job, location and period of employment.”

Lawyers for Wal-Mart say the lower court decisions in the case are rewriting employment law to allow class action lawsuits for money damages under a rule, they say, that only permits judges to order corrective action.

They also argue that the class action process will undercut Wal-Mart’s ability to defend itself against charges that it intentionally discriminated against each of the 1.5 million women.

“The class is larger than the active-duty personnel in the Army, Navy, Air Force, Marines, and Coast Guard combined – making it the largest employment class action in history by several orders of magnitude,” wrote Theodore Boutrous in a legal brief filed on behalf of Wal-Mart.

Lawyers for the women are expected to file a reply brief, urging the court not to take up the case and allow the lower court ruling to stand.

A decision by the justices on whether to take up the case is not likely until next year.