California activist Jim Steyer is following his billionaire little brother into national politics — he’s launching a political advocacy group, and a super PAC may follow. Steyer, 57, a children’s advocate with close ties to Hillary Clinton, hasn’t quite figured out how to stage his debut as a major player. But he promises it will be memorable. “You don’t bring a squirt gun to a fight where the other guys have AK-47s,” he said in a telephone interview. “I will tell you this: We’re fearless.” Younger brother Tom Steyer, 56, splashed onto the national radar last fall with huge investments...