SAN FRANCISCO — The federal government got off the sidelines Thursday to throw a few blocks for plaintiffs in a huge gender class action against Wal-Mart.

The move is a reversal by the U.S. Equal Employment and Opportunity Commission, which had previously decided to sit the case out. Instead, the EEOC now opposes Wal-Mart’s bid to try punitive damages on a case-by-case basis, according to an amicus curiae brief the commission filed with the Ninth Circuit U.S. Court of Appeals.