Rose Ann DeMoro, a former supermarket cashier from the suburbs of St. Louis who has risen to become one of America’s most powerful labor leaders, recalls that her tough but saintly mother had two maxims. One: “Keep reaching.†Two: “If someone hits you, and you don’t hit ’em back, you’re going to get it when you get home.†DeMoro, executive director of the 86,000 member California Nurses Association and the 155,000-member National Nurses United, says she has faithfully lived by the first — and never had to worry much about the second.