At issue is Boeing's fractious relationship with Local 751 of the International Assn. of Machinists and Aerospace Workers, which represents employees at Boeing's plants around Seattle and Portland, Ore. The union has gone on strike five times since 1975, most recently in 2008. Although the company and the union settled their differences and agreed to a four-year contract later that year, in 2009 Boeing announced that it would create a second production line for its new 787 Dreamliner in North Charleston, S.C., a right-to-work state, instead of at the Puget Sound plants that were already building the plane.