Last-minute talks were underway between the Boeing Co. unit that operates the Delta rocket program and the union representing about 1,500 machinists as a midnight strike deadline approached.
Two weeks ago, locals with the International Assn. of Machinists and Aerospace Workers in California, Alabama and Florida rejected Boeing's latest offer for a three-year contract.
Terms of the proposal were not disclosed.
The union later delivered notice to Boeing of its intent to strike at 12:01 a.m. today. A federal mediator was meeting with both sides Tuesday.