Amazon.com Inc. workers at an Alabama warehouse will vote by mail in February and March on whether to form a union, the National Labor Relations Board said, setting a date for a closely watched referendum on the relationship between the largest online retailer and the employees who pack and ship its products.

A group of about 6,000 frontline employees at the fulfillment center in Bessemer, Alabama, will decide whether to join the Retail, Wholesale and Department Store Union, the NLRB said Friday. Typically, such votes are held in person or at a location close to the workplace. Elections since the start of the Covid-19 pandemic have been conducted by mail.

Ballots will be mailed to eligible workers at the Bessemer facility on Feb. 8, and must be received by the NLRB’s regional office by March 29, the agency said in its decision. The federal labor regulator will begin counting the following day.

Amazon, the second-largest U.S. employer behind Walmart Inc., has largely avoided unions in its ranks, though some of its workers in Europe are members of labor groups. The vote is Amazon’s first in the U.S. since 2014, when a small group of technicians at a Delaware warehouse voted against joining the International Association of Machinists and Aerospace Workers.

Amazon said it disagreed with the NLRB’s decision. “We believe that the best approach to a valid, fair and successful election is one that is conducted manually, in-person,” spokesperson Heather Knox said in an email. “We will continue to insist on measures for a fair election, and we want everyone to vote, so our focus is ensuring that’s possible.”

Amazon’s hundreds of thousands of U.S. warehouse workers found themselves in the spotlight in the last year as the pandemic sparked a surge in online shopping. Some workers criticized the company for what they said was a lack of safety measures amid outbreaks in several facilities. The company fired several workers who went public with such critiques. In a December complaint, an NLRB regional director accused Amazon of terminating a Staten Island warehouse worker because he participated in a protest calling for better protection from the pandemic.

Amazon, which went on a hiring spree to help meet the rush of orders, said it has worked to keep its employees safe. It has denied retaliating and said the dismissed employees violated company policy.

Dueling websites foreshadow the war of words over the issue in the coming months. A union site, featuring testimonials from workers about changes they’d like to see at Amazon, has been up for months. It’s now joined by doitwithoutdues.com, which bears the Amazon logo and criticizes union dues. Amazon’s Knox said the company created the site to help “employees understand the facts of joining a union.”

“If the union vote passes, it will impact everyone at the site and it’s important associates understand what that means for them and their day-to-day life working at Amazon,” she said. Amazon uses the term associates for its hourly employees.

A spokesperson for the RWDSU declined to comment on Friday’s decision.

Amazon had argued in hearings in December that despite the pandemic’s uncontrolled surge in Alabama, the vote should be held in person. Harry Johnson, a former Republican NLRB member who is representing Amazon, made the case the company’s facility was safer than surrounding Jefferson County.

“Given the prevalence of asymptomatic transmission and the presence of Covid-19 both inside and outside the Employer’s facility, the overall state of crisis in Jefferson County cannot be ignored,” the NLRB’s acting regional director wrote in Friday’s decision.