Boeing worker Paul Burton holds a picket sign and yells during a rally Wednesday at Machinists union headquarters in Seattle. Although union members voted Wednesday to authorize a strike, union negotiators extended the contract 48 hours.

SEATTLE — Boeing aircraft assembly workers voted to strike for an unprecedented second time in three years, but their contract was extended 48 hours at the request of Washington Gov. Chris Gregoire and a federal mediator.

The vote late Wednesday was 87 percent in favor of a strike as unanimously recommended by machinists union negotiators. Under union rules, a strike requires at least two-thirds support from those voting.

In separate balloting, union members also voted 80 percent to reject Boeing's third and final three-year contract offer, which included pay raises averaging 11 percent.

The walkout was to begin at 12:01 a.m. PDT today after the expiration of a contract covering more than 27,000 workers but will have to wait because of the contract's short extension.