Boeing Co. got a boost from the National Labor Relations Board in the aircraft maker’s legal challenge to a union’s election victory at its South Carolina plant.

The NLRB on Sept. 7 agreed to accept friend-of-the-court briefs backing Boeing filed by four Republican governors, the U.S. Chamber of Commerce, and a slew of other business groups.

Boeing is fighting the International Association of Machinists’ win in a May 31 union election to represent a small group of technicians at the company’s facility in North Charleston, S.C. Boeing says the Machinists “gerrymandered” the bargaining unit—composed of about 170 of the 7,000 workers at the plant. That smaller union election came after a larger unit at the plant rejected the union in a 2,087-731 vote in 2017. The union previously withdrew an election petition in 2015.

The Machinists’ campaign at Boeing’s South Carolina plant represents a high-profile test of whether organized labor can make headway in “right-to-work” states via small bargaining units, also known as a “micro-units.” South Carolina is one of 27 states with right-to-work laws that prohibit agreements requiring workers to unions or pay certain fees as a condition of employment.

Boeing says an NLRB regional director improperly approved the small unit of flight-readiness technicians. The aircraft manufacturer says that ruling was incorrect under the labor board’s 2017 decision in PCC Structurals that re-established a traditional “community-of-interest” test for determining whether units are appropriate. That decision overturned the NLRB’s 2011 ruling in Specialty Healthcare, which had made it easier for unions to win approval for smaller bargaining units.

The governors of South Carolina, Maine, Kentucky, and Mississippi echoed Boeing’s argument in their brief, saying the regional director’s decision looked like an effort to dodge the PCC Structurals standard and “smuggle” in the old test from Specialty Healthcare.

The Machinists had opposed the NLRB letting the governors and the business groups file briefs, saying that they just repeated arguments Boeing had already made to the board.

The board didn’t explain why it decided to allow the briefs. It gave the union until Sept. 21 to file a response.

The case is The Boeing Co., N.L.R.B., No. 10-RC-215878, order granting leave to file amicus briefs 9/7/18.