A strongly contested union election pits the world's largest aerospace company against the International Association of Machinists and Aerospace Workers on Wednesday.
Three thousand workers at Boeing's plant in South Carolina are eligible to vote on whether they want to be represented by the union.
Many businesses and key political leaders in South Carolina have spoken against the union. Union membership in the conservative state is just 1.6 percent, the lowest in the nation. The low level of union representation is said to be one reason that Boeing located the plant in the state.
Boeing employs about 7,500 people in South Carolina, and nearly 150,000 around the world. The company opened a plant in South Carolina after labor strife in Washington state, where unions have been more successful.
The plant is one of two that makes Boeing's 787 commercial jetliner.
The union vote follows extensive organizing efforts by union supporters and a series of posters, meetings and television commercials by the company urging employees to vote against the union.
Published reports say President Donald Trump may visit the plant on Friday, after the vote.