Leaders of the International Association of Machinists and Aerospace Workers endorsed a contract proposal from Boeing Co. yesterday, greatly reducing the chances that its 44,000 members will strike on Thursday.

After 12 days of intense negotiations at a hotel outside Seattle, Boeing submitted its final contract proposal to the machinists early yesterday, offering several concessions and a wage increase of nearly 16 percent over three years.

"We believe this contract is the best contract in aerospace, period," said Bill Johnson, a union negotiator who is also president of the Seattle local that represents 31,000 Boeing workers. "They gave us everything we wanted."

But the endorsement of the leaders of the union, which represents nearly one-quarter of Boeing's 207,000 workers, does not guarantee that a strike will be averted. The union leadership also backed a company proposal in 1995, only to see it rejected by the workers, leading to a 69-day strike.

Members will vote on the proposal on Wednesday, the last day of their 44-month contract. A majority is needed to approve the offer. If that vote fails, a two-thirds majority is needed to initiate a walkout.

A strike would cripple the Seattle-area factories that produce Boeing's flagship commercial jets at a time when the company is beginning to recover from a production foul-up that forced it to halt production for a month in 1997 and caused the company to post its first annual loss in 50 years.

"I think that we have a contract we can be proud of," Philip M. Condit, Boeing's chairman and chief executive, said in an interview yesterday.