More than 1.5 million American women who’ve worked for Wal-Mart in recent years joined together to file a class-action lawsuit against the behemoth company. The case certainly appeared credible: the women had evidence that Wal-Mart favored men over women when it came to compensation and promotions, and filed suit to end the discrimination and recover lost wages.

The case has been in the courts for years, but has never been heard on the merits. Rather, the company’s lawyers have argued that the women shouldn’t even be able to join together to file the lawsuit in the first place. Yesterday, the U.S. Supreme Court weighed in.

Dahlia Lithwick, who said the Wal-Mart v. Dukes case is likely “the most consequential” of the term, explained why the high court’s five conservatives sided against the plaintiffs.

Wal-Mart, the nation’s largest private employer, seems to have figured out that the key to low-cost discrimination lies in discriminating on a massive scale. […]

The red meat of the Wal-Mart decision lies in the fight between Scalia and Justice Ruth Bader Ginsburg over a much more fundamental question: Was there a single question of law or fact common to all the women in the suit? The federal district court and 9th Circuit believed that there was. The five justices in the majority disagreed.

Scalia concludes that (even in advance of a lawsuit) the women could not show that Wal-Mart “operated under a general policy of discrimination.” That’s partly because “Wal-Mart’s announced policy forbid sex discrimination” and partly because he rejects the plaintiffs’ claim that Wal-Mart’s “policy” of allowing discretion by local supervisors over employment matters constitutes a policy at all. As Scalia sees it, in giving local managers so much leeway in making personnel decisions, Wal-Mart actually established “a policy against having uniform employment practices.” It’s not Wal-Mart discriminating against women. It’s just all these men doing it, and God knows men don’t have unconscious biases and prejudices against women.

Making matters worse, the 5-4 ruling did more than just shield Wal-Mart. The court’s conservatives rejected the women’s class-action case and then went further, making it more difficult for others to file class-action suits in the future. As the NYT editorial board explained:

For 45 years, since Congress approved the criteria for class actions, the threshold for certification of a class has been low, with good reason because certification is merely the first step in a suit. Members of a potential class have had to show that they were numerous, had questions of law or fact in common and had representatives with typical claims who would protect the interests of the class.

Justice Scalia significantly raised the threshold of certification, writing that there must be “glue” holding together the claims of a would-be class. Now, without saying what the actual standard of proof is, the majority requires that potential members of a class show that they are likely to prevail at trial when they seek initial certification. In this change, the court has made fact-finding a major part of certification, increasing the cost and the stakes of starting a class action.

This was not a good day at the Supreme Court.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.