There hadn't been a nurses' union in Texas since the 1970s, until Dallas-based Tenet Healthcare Corp. opened the door to nurses' unionization when Cypress Fairbanks Medical Center in Houston had the first successful union vote in the state's history. Now the California Nurses Association is hoping that agreement will give it enough leverage to push its way into Dallas-Fort Worth, where Tenet owns three hospitals. From there, the union says, it plans to head into unaffiliated hospitals throughout the state to bring Texas in line with other states where nurses are organized.