ELECTION 2006 Unions threaten consumer boycott to dry up campaign contributions California labor groups warn national retailers: Oppose Reiner initiative, you'll be blackballed Union supporters of a California ballot measure promising "Preschool for All" have threatened several national retailers with a nationwide boycott if their companies or major shareholders make contributions to the opposition campaign. Representatives for the Gap, Old Navy and Banana Republic – major mall-based clothing retailers – were sent letters last week from the Service Employees International Union and the American Federation of State, County and Municipal Employees warning that a national action against all their stores...