The union representing thousands of striking Boeing (BA.N), opens new tab U.S. factory workers said late on Tuesday a survey showed members were overwhelmingly against the planemaker's latest pay proposal, which it described as its "best and final".

Earlier in the day, Boeing said it would extend the timeline for a vote on a new contract after the union had rejected the company's initial Friday deadline.

More than 32,000 Boeing workers in the Seattle area and Portland, Oregon, walked off the job on Sept. 13 in the union's first strike since 2008, halting production of airplane models including its best-selling 737 MAX.

Nearly 95% rejected Boeing's offer of a 25% pay rise over four years, leading to the strike. An improved proposal announced by Boeing on Monday would lift the pay gain to 30% and restore a performance bonus, but the union said a survey of its members found that was not enough.

"The survey results from yesterday were overwhelmingly clear, almost as loud as the first offer: members are not interested in the company's latest offer," International Association of Machinists and Aerospace Workers District 751 said on Tuesday.

The union has been seeking a 40% pay rise and the restoration of a defined-benefit pension that was taken away in the contract a decade ago.

Boeing, which did not immediately comment, put forward the latest offer on Monday without negotiating with the union on it beforehand. The union declined to put it to an immediate vote, saying it would survey its members before deciding.

"We heard you, and you've told us loud and clear that this proposal did not go far enough to address our members' priorities," the union said on Tuesday. "We have made it clear that we are ready to schedule mediated or direct talks with Boeing as the path to find a resolution to this strike."

BACKTRACKING

Labor experts said Boeing offer to move the deadline for a vote was the latest case of backtracking, which could undermine the planemaker's credibility.

Before the strike, Boeing's commercial planes chief Stephanie Pope told workers that its proposal at that time was the best deal they would get, yet on Monday it made an improved "best and final" offer.

"It's not a great thing for Boeing to say this is our final offer and then fairly quickly reverse," said Harry Katz, a professor of collective bargaining at Cornell University’s School of Industrial and Labor Relations.

Striking workers are set to miss their first paychecks on Thursday.

Katz said workers feeling the pinch of missing wages should consider Boeing's latest offer, which he called solid.

The strike is the latest event in a tumultuous year for Boeing that began with a January incident in which a door panel detached from a new 737 MAX jet mid-air.

During the strike, Boeing has frozen hiring and started furloughs for thousands of U.S. employees to reduce costs. Boeing has planned for non-union workers to take one week of furlough every four weeks on a rolling basis for the duration of the strike.

According to economic data analytics firm IMPLAN, if Boeing's strike continues through Sept. 27, it would reduce U.S. gross domestic product by $1 billion, and lead to $500 million in lost labor income.