The union seeking to organize 3,175 Boeing Co. jet assembly staff at the company's nonunion South Carolina factories said it will decide by week's end if it will cancel a planned April 22 vote, a union official said Tuesday.

Boeing's largest union, International Machinists and Aerospace Workers, sparked a fresh confrontation with the company in March when it filed papers with the National Labor Relations Board to unionize nearly half of the company's staff in the state.

Withdrawal of its petition for an election at Boeing's South Carolina facilities has "always been a possibility" if sufficient support didn't materialize during the organizing effort and cancellation is "being given serious consideration," said IAM spokesman Frank Larkin.

Mr. Larkin said 125 to 150 union staff and volunteers, including "several dozen" local Boeing employees are canvassing the eligible staff with phone calls and home visits to gauge support for the creation of a collective bargaining agreement. Mr. Larkin said it was not unusual for an organizing effort to take two or three attempts before a collective bargaining unit was created and said any cancellation would not signal an end to its local campaign.

The measurement of support, said Mr. Larkin, comes as the union claims misinformation about moving the factory or losing future work has undermined its organizing effort. While the company is barred from retaliating against employees for any unionization vote, "rumors on the shop floor can come without clear attribution," he added.

A Boeing spokeswoman did not immediately respond to a request for comment on Tuesday. Previously, Boeing has vocally opposed any effort to unionize its staff in South Carolina.

Many local politicians including South Carolina Governor Nikki Haley and North Charleston, S.C., Mayor Keith Summey have been vocal in their opposition of the unionization drive.

Boeing has quickly expanded its presence in South Carolina, where it employed around 7,700 people as of March 26. Staff there build major portions of its advanced long-range 787 Dreamliner and the company currently assembles three Dreamliners each month at its sprawling North Charleston plant and plans to accelerate to seven per month later in the decade.

Curtailing the April 22 vote, which coincides with the release of Boeing's quarterly earnings disclosure, would require the union again to collect signed cards from at least 30% of eligible staff indicating a desire to vote on unionization. A new petition to the NLRB to hold a fresh vote would have to wait six months.

A spokeswoman for the labor board did not immediately respond to a request for comment.

Write to Jon Ostrower at jon.ostrower@wsj.com

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