SAN FRANCISCO — The Ninth Circuit U.S. Court of Appeals voted 6-5 Monday to certify a gender discrimination claim against retail behemoth Wal-Mart, which plaintiff lawyers say is the largest employment discrimination class action ever.

The case involves more than a million potential plaintiffs and is widely expected to draw U.S. Supreme Court review. Not only was the Ninth Circuit split along ideological lines, but the Roberts court has shown a drive to tackle threshold questions of civil procedure that determine whether a case goes forward, said Douglas Hallward-Driemeier, a Ropes & Gray partner in Washington, D.C., and former assistant to the solicitor general.