The strike by about 3,200 machinists at the plants in Mascoutah, Illinois, and the Missouri cities of St. Louis and St. Charles is smaller in scale than a walkout last year by 33,000 Boeing workers who assemble commercial jetliners but threatens to complicate the aerospace company’s progress in regaining its financial footing.

“Boeing claimed they listened to their employees – the result of today’s vote proves they have not,” Brian Bryant, president of the International Association of Machinists and Aerospace Workers, said in a statement.

Boeing said it was disappointed by the result and noted the vote had been a close one. The company said in a statement that it was increasingly hearing from workers “who want to cross the picket line” and “understand the value of our offer.”

“The union’s statement is misleading since the vote failed by the slimmest of margins, 51% to 49%,” the statement read. “We are turning our focus to executing the next phase of our contingency plan in support of our customers.”

The machinists’ union acknowledged the vote was close but said in a message to members that “very few” workers have crossed the picket line.

“Our solidarity remains strong, and the company’s claim otherwise is wrong,” the union said.

Union leaders say talks have stalled over issues such as wages and retirement benefits, while Boeing has argued that workers’ demands exceed the cost of living in the Midwest.

Ahead of Sunday’s vote, the union told its members that it did not recommend approval of the company’s latest offer, which it said “had no meaningful improvements” to retirement benefits and wage increases for workers with more seniority.

Negotiations escalated over the summer in the days leading up to the strike, with the workers rejecting an earlier proposed agreement that included a 20% wage hike over the life of the five-year contract.

Boeing quickly countered with a modified agreement that didn’t boost the proposed pay raises but did remove a scheduling provision affecting the workers’ ability to earn overtime pay. Workers rejected that offer, too, and went on strike the next morning. They also voted against revised terms in September.

The company has said that it was prepared for a strike, with a contingency plan in place “to ensure our non-striking workforce can continue supporting our customers.”

Boeing’s Defense, Space & Security business accounts for more than one-third of the company’s revenue. Boeing is set to report its third-quarter earnings on Wednesday.