The 9th U.S. Circuit Court of Appeals voted 6-5 Monday to certify a gender discrimination claim against retail behemoth Wal-Mart, which plaintiffs 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 9th 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.