SEATTLE (AP) -- A last-minute move to avert a potentially bruising strike that could cost more than 27,000 workers weeks of pay would usually be met with unalloyed relief.

Not with the Machinists union faithful Wednesday night after Boeing Co. aircraft assembly workers voted overwhelmingly to strike for an unprecedented second time in three years, then learned both sides had agreed to a 48-hour contract extension at the request of Washington Gov. Chris Gregoire and federal mediators.

Chief negotiator Mark Blondin and Tom Wroblewski, president of Machinists District Lodge 751, were repeatedly shouted down at the union hall with catcalls of "Sellout!" and "What was the strike vote for?"

For Boeing, the rejected offer took "the best contact in the industry and we made it better," Vice President of Human Resources Doug Kight said in a news conference.

He repeatedly turned aside questions of whether Boeing was relieved to get another chance to avert a strike by offering a sweeter deal.

"Responding to a request from the federal mediators is appropriate for both sides," Kight said. "My job at this point is to listen to the union."

Jim Proulx, a Boeing spokesman, said company and union negotiators would meet together with a federal mediator at an undisclosed time and location Thursday.

Boeing's "best and final offer," presented last Thursday after talks that began May 8, included bonuses totaling at least $5,000 and averaging $6,400, 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.

The International Association of Machinists and Aerospace Workers represents about 25,000 workers in District 751 in the Seattle area, 1,500 in District 24 in Gresham, Ore., a Portland suburb, and about 750 who do military work for Boeing in Wichita, Kan.

Analysts say a strike could cost Boeing about $100 million per day in deferred revenue. During the last strike, one of the shortest in company history, Boeing was unable to deliver more than two dozen airplanes on schedule.

In a vote that hinged largely on approximately 7,000 union members who were hired since the Machinists' last strike against Boeing -- a 24-day walkout in 2005 -- 80 percent voted no on the offer and a whopping 87 percent elected to go on strike if the offer was rejected, far more than the two-thirds required for a walkout.

With anything less than two-thirds for a strike, under union rules, the offer would have taken effect by default regardless of the vote to reject it.

The extension pushed back the deadline for a walkout from 12:01 a.m. Thursday to the same time Saturday.

With the Machinists' membership split between the post-'05 hires and seasoned veterans of a dozen years or more and few in between, union leaders accused Boeing of conducting a media blitz focusing on recent hires as the most likely prospects to vote against a strike.

"They've got 48 hours to bring a deal acceptable to you," Blondin told more than 100 shop stewards and others who had been chanting "Strike, strike, strike!"

Struggling to make himself heard, a task the older Wroblewski found nearly impossible, Blondin insisted that it was worth one more try to reach agreement at the bargaining table without a strike.

"We have told you all along that our job as negotiators is to negotiate a contract that is acceptable to you, not to negotiate a strike," Blondin said.

Confronted on his way out of the room by Randy Carroll, a mechanic in Auburn, he said union negotiators would not accept continued extensions.

"If we can't do it in 48 hours, brother, it's on," Blondin said.