SEATAC, Wash. -- Boeing Co. aircraft assembly workers voted Wednesday on whether to strike for an unprecedented second time in successive contract cycles.

Polls closed at 6 p.m. and the 20,000-plus ballots will be counted from the Puget Sound area. Adding those to the Wichita and Portland units will take several hours, and a result is not expected until 8:30 or 9 p.m.

In a high-stakes struggle between the Chicago-based aerospace giant and the Machinists union, 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.

Union leaders countered with the slogan, "It's our time this time," saying the offer came up far short in pay, retirement benefits, health care, limits on outsourcing and other provisions.

Negotiators for the International Association of Machinists and Aerospace Workers, representing 25,000 employees in and around Seattle, about 1,500 in Gresham, Ore., a Portland suburb, and about 750 who do military work for Boeing in Wichita, Kan., unanimously recommended rejection of the offer on one ballot and approval for a strike on another.

Under union rules, at least two-thirds approval is required for a strike. With anything less, a contract takes effect regardless of the vote on the offer.

A $2,500 signing bonus, however, hinged on majority approval of the contract as well as a lack of sufficient votes to call a strike.

Thus, if a contract takes effect without a majority vote to accept the offer, Boeing would avoid paying $2,500 per employee, reducing the immediate cost of the new contract by more than $67.5 million -- about the top-end list price of a new 737-700 passenger jet.

Either way, Boeing would still pay a separate lump sum of at least $2,500 or 6 percent of each employee's previous year's earnings, including overtime, an average of $3,900.

A walkout would begin at 12:01 a.m. Thursday, when the current pact expires, and would be first in successive IAM contract talks covering riveters, electricians, painters and other workers who assemble commercial jets and key components at Boeing.

Strikes in 1948, 1965, 1977, 1989 and 1995 were each followed by worker approval of a contract in the next round of bargaining. The Machinists' last strike against Boeing ended after 24 days with ratification of a three-year contract in 2005.

Facing about an eight-year backlog of deliveries, potential losses as high as $100 million a day in deferred revenue and further delays in delivery of the new 787 commercial jet, Boeing campaigned hard for the offer in workplace meetings and a media blitz of radio, television and newspaper ads.

Union leaders pressed their case for a strike vote in rallies and marches.

Last week the union filed an unfair labor practice charge with the National Labor Relations Board, accusing Boeing of breaking federal law by interrogating workers one-on-one about the bargaining and the vote.

Company officials said managers had been asked to relay worker comment and reaction to the offer from the shop floor and denied breaking the law or doing anything differently from past negotiations.