The union's roughly 33,000 Seattle-area workers are casting separate votes on whether to accept the contract and whether to strike, with the polls scheduled to close Thursday evening.
A strike could begin after midnight on Friday. Led by new CEO Kelly Ortberg, the embattled aviation giant had hoped a 25-percent wage hike over four years and a commitment to invest in the Puget Sound region would avert a costly strike.
But while the preliminary contract won an endorsement from leaders of the International Association of Machinists and Aerospace Workers (IAM) District 751, the response from much of the rank-and-file has been harsh.
Workers had sought a 40-percent wage hike and critics have said the 25 percent figure is inflated because the new deal also eliminates an annual company bonus.
Other points of contention include the deal's failure to restore a pension and a Boeing pledge to build its next plane in the Seattle region that critics view as "hollow."
TV reports in the Seattle region have featured footage of line workers marching in solidarity against the deal.
"Bad Deal," reads a flier that has been posted to social media.
Ortberg implored workers Wednesday to not strike and "sacrifice" future progress over "frustrations of the past."
"For Boeing, it is no secret that our business is in a difficult period, in part due to our own mistakes in the past," Ortberg said.
"Working together, I know that we can get back on track, but a strike would put our shared recovery in jeopardy, further eroding trust with our customers and hurting our ability to determine our future together."
A strike would shutter Boeing production assembly plants for the 737 MAX and 777, further delaying the company's turnaround efforts.
"We have achieved everything we could in bargaining, short of a strike," IAM President Jon Holden said in a message to workers.
"We recommended acceptance because we can't guarantee we can achieve more in a strike," Holden said. "But that is your decision to make and is a decision that we will protect and support, no matter what."
If the contract fails to win a majority but a strike vote also falls short, the contract offer is accepted by default, according to IAM rules.
In an interview with the Seattle Times published Monday, Holden said, "Right now, I think it will be voted down, and our members will vote to strike."
Advantage: labor?
Boeing has been under renewed scrutiny since a January incident in which a fuselage panel blew out of an Alaska Airlines Boeing 737 MAX plane mid-flight, necessitating an emergency landing.
That revived questions about safety and quality control after the company had seemingly made progress following deadly MAX crashes in 2018 and 2019.
The aerospace giant in March announced a management shakeup that included the exit of Dave Calhoun as CEO. It has also slowed production on the MAX as it beefs up quality control.
Ortberg, who took the helm on August 8, has pledged a "reset" on labour relations as part of a turnaround.
The IAM talks come on the heels of a more assertive labour movement as embodied by strikes at Detroit's "Big Three" automakers and John Deere, and a near-strike at UPS that was resolved with a last-minute deal with the Teamsters.
"The power balance has shifted in favour of workers," said Cornell University labour relations expert Harry Katz, who noted that Boeing's position has been weakened by "turmoil and management problems."