Boeing Co. aircraft assembly workers voted overwhelming to reject the company's contract offer and strike for an unprecedented second time in three years.

Their walkout was set to begin at 12:01 a.m. PDT Thursday but was put on hold after their expiring contract — covering more than 27,000 workers — was extended 48 hours at the request of Washington Gov. Chris Gregoire and a federal mediator.

Machinists District Lodge 751 President Tom Wroblewski and chief machinists' negotiator Mark Blondin said, however, that if Boeing doesn't come back with an acceptable deal within 48 hours, the strike is on.

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.

Boeing's three-year "best and final" offer included bonuses totaling at least $5,000, raises averaging 11 percent, pension increases and a 3 percent cost-of-living adjustment — $34,000 in average pay and benefit gains per employee, according to the company.