The union hoping to change US labour history by organizing Amazon.com warehouse employees in Alabama faces a steep uphill slog when vote counting resumes on Friday.
Amazon workers at the warehouse in Bessemer were on track to reject unionization by a 2-1 margin, with almost half the 3,215 ballots counted on Thursday. Some 1,100 ballots were voted against forming a union, with 463 ballots in favour.
The vote count will resume at 8:30 a.m. CT (1330 GMT) Friday.
Unionizing Amazon, the second-largest private employer in America, could be a start to reverse long-running declines in union membership, which fell to 11% of the eligible workforce in 2020 from 20% in 1983, according to the US Bureau of Labor Statistics.
Whatever the results, the Retail, Wholesale and Department Store Union (RWDSU), which is trying to organize the employees, has the same legal options as Amazon: challenge the eligibility of individual voters or allege that coercive conduct tainted the election.
In the latter case, the dispute would play out before the National Labor Relations Board (NLRB) and then likely in a federal appeals court.
The vote count followed more than a week of challenges to ballots during closed-door proceedings that could influence the final result. Lawyers for Amazon and the union were allowed to question ballots on suspicion of tampering, a voter's eligibility and other issues.
The union says there have been hundreds of contested ballots, making it unclear the number of votes needed to declare a winner.
The NLRB, which is overseeing the election, would adjudicate challenges in coming days.
Amazon for years has discouraged attempts among its more than 800,000 US employees to organize, showing managers how to identify union activity, raising wages and warning that union dues would cut into pay, according to a prior training video, public statements and the company's union election website.
Amazon has said it is following all NLRB rules and wants employees to understand each side of the contest, and that the RWDSU does not represent a majority of its employees' views. The company has said it wants as many of its employees to vote as possible.