NORTH CHARLESTON, S.C. — Boeing came to South Carolina more than seven years ago to establish a second assembly line for its 787 Dreamliner aircraft. At least part of the attraction, analysts said, was the area’s lightly unionized labor force — giving the company more leverage over the union at its main operations outside Seattle.

Now that equation is being put to the test. Workers here will vote Wednesday on whether to unionize, an early test of organized labor’s strength in the Trump era.

A vote to form the union would be a major victory for the International Association of Machinists and Aerospace Workers, which the workers would join.

“If Boeing loses, it removes the ability to hold nonunion Charleston over the heads of the union in Seattle, to beat up that union for more concessions in the future,” said Scott Hamilton, the managing director of Leeham Company, an aviation analysis firm in Bainbridge Island, Wash.

The stakes appear even higher amid reports that President Trump will visit the South Carolina plant on Friday. Mr. Trump has criticized Boeing over the cost of a new Air Force One project, but it is also a major employer and exporter, as well as a military contractor — one that Mr. Trump has pitted against Lockheed Martin in seeking price savings on fighter jets.

Boeing has roughly 7,500 workers in South Carolina, primarily in North Charleston and Ladson, of whom a little over 3,000 are eligible for the union. There are about 30,000 unionized workers at its facilities in the Puget Sound area, where it turns out other aircraft models as well.

The machinists made a similar union push here nearly two years ago, but called off a vote a several days before the scheduled election, complaining of a deluge of misinformation from management and pressure from local politicians.

Two Boeing officials, who would not speak for attribution, expressed confidence that the company would turn back the unionization effort and suggested that the union was racing to act before Mr. Trump appoints two members to vacant seats on the National Labor Relations Board, which oversees union elections, and overturns its Democratic majority. If the union were to win, Boeing would have to file any objections to a regional official of the agency within days, with any subsequent appeal going to the board in Washington.

Mike Evans, the machinists’ lead organizer in South Carolina, rejected that suggestion, saying the union believes it has the votes to prevail.

“If we weren’t ready, we’d still be down here sharing information, getting up the facts of the benefits of collective bargaining,” he said. “We feel we’ve reached that and have a significant amount of support. That’s what dictates, ‘Why now?’”

Under rules overseen by the labor board, unions can call an election once at least 30 percent of eligible workers have signed up to authorize one, but the unions typically wait until they have a substantially higher level of support to improve their odds of success.

The machinists may also benefit from two years’ worth of accumulated frustrations among workers. Mr. Evans said workers felt their evaluations were frequently arbitrary, an issue that the company promised to address before the first scheduled vote.

“Whatever changes were promised it’s been more of the same,” AJ Russo, a production coordinator at the Boeing plant in North Charleston, said in an interview after a pro-union rally Monday, at which he spoke. How workers are treated “depends on where you’re standing, what day it is.”

The issue is all the more salient because the company recently announced it would undergo a round of job reductions that will affect its South Carolina operations amid intense competition from its chief rival, Airbus. Management has not ruled out involuntary layoffs if the company is unable to hit its goal for cost savings. While the cuts discussed so far would not affect union-eligible workers, they are a source of anxiety, Mr. Evans said.

In addition to making the evaluation and layoff process fairer in the eyes of workers, a union could bargain for higher wages. The average wage for unionized Boeing workers in Washington State is about $31 per hour, versus about $23 per hour for union-eligible workers in South Carolina, according to data from the company and the union.

Boeing has said the difference is driven partly by the local cost of living and labor market conditions, and partly by the longer average tenure of workers in Washington. The union says pay tops out at a certain point, making additional tenure relevant only to issues like benefits and job security, not wages.

Even if the union prevailed, it wouldn’t necessarily restore the leverage it enjoyed before the Dreamliner plant opened here. South Carolina is a “right to work” state, meaning workers do not have to join the union or pay union fees as a condition of employment.

According to Richard L. Aboulafia, an aviation analyst at the Teal Group in Fairfax, Va., this was an important consideration in Boeing’s decision to locate here.

“If you have just enough folks who will cross the picket lines, you can still get certification and testing work done,” Mr. Aboulafia said, alluding to the possibility of a strike. “That can make a difference in keeping some revenue coming in.”

Mr. Evans and his fellow organizers appear to have some other advantages over the 2015 effort, when local political leaders campaigned aggressively against the union.

Nikki Haley, then South Carolina’s governor and now the United States ambassador to the United Nations, portrayed the machinists’ union as a group of meddling outsiders who had resisted Boeing’s move to South Carolina.

“I want our work force to remember that if it were up to the I.A.M., there would be no Boeing in Charleston,” Ms. Haley said at a campaign stop a few weeks before the scheduled 2015 unionization vote, according to the local Post and Courier.

She was alluding to the union’s charge that Boeing was setting up the plant to undermine the right of workers in Washington to strike.

This time, the political powers that be appear to have been largely silent.

In addition, Mr. Evans said the company had been more forthcoming with information about eligible workers than it was in 2015, making outreach easier. Under N.L.R.B. rules enacted later that year, companies are required to hand over the names of union-eligible workers earlier in the election process.

Still, the campaign faces considerable obstacles. A local business group tied to the South Carolina Manufacturers Alliance, of which Boeing is a member, has been running advertisements on local television, including one depicting a thuggish casino boss urging people to roll dice at a craps table. The message is that the union wants workers to gamble away what they already have.

Boeing implies that a vote to unionize would mean inserting the union into the relationship between workers and managers, something the union says is false. Organizers say that union stewards would be available when workers felt they had been mistreated, but that otherwise workers would be left to interact with managers as they saw fit.

The company has also sought to play up a split between the union and the South Carolina workers by invoking the union’s efforts to preserve production in Washington.

“We have not forgotten the I.A.M.’s history in South Carolina, including their repeated insults regarding our teammates’ abilities,” said Joan Robinson-Berry, the general manager of Boeing South Carolina, in a statement after the union vote was announced in January.

And then there is the general challenge of South Carolina’s political culture, famously hostile to unions.

“I taught undergrads here at the university for many years,” said Hoyt N. Wheeler, an emeritus business professor at the University of South Carolina who taught labor relations and employment law. “The local students are violently opposed to unions. They knew absolutely nothing about them, but they had strong opinions.”

If the vote succeeds, Mr. Wheeler said, it could go a long way toward demystifying unions in the state.

“One of the things that happens in a state like South Carolina where you don’t have organized workers is it’s hard for people to learn anything about a union, what it is,” he said.

“You don’t have neighbors, friends, relatives in a union,” he added. “It would be good to have an example like that in South Carolina.”