As Amazon workers in Alabama debate unionizing, The Times investigates how the company has tried to thwart past efforts.

Amazon is facing the largest and most viable U.S. labor challenge in its history: a union vote at a warehouse in Bessemer, Ala.

Nearly 6,000 workers have until March 29 to decide whether to join the Retail, Wholesale and Department Store Union. A labor victory could energize workers in other U.S. communities, where Amazon has more than 800 warehouses employing more than 500,000 people.

“This is happening in the toughest state, with the toughest company, at the toughest moment,” said Janice Fine, a professor of labor studies at Rutgers University. “If the union can prevail given those three facts, it will send a message that Amazon is organizable everywhere.”

Over the last two decades, as the internet retailer mushroomed from a virtual bookstore into a $1.5 trillion behemoth, it has forcefully — and successfully — resisted employee efforts to organize. Some workers in recent years agitated for change in Staten Island, Chicago, Sacramento and Minnesota, but the impact was negligible.

But the most recent campaign is gaining momentum, and received a shot in the arm last month, when President Biden weighed in.

“There should be no intimidation, no coercion, no threats, no anti-union propaganda,” he said in a video posted on Twitter that never mentioned Amazon but referred to “workers in Alabama” deciding whether to organize a union. “You know, every worker should have a free and fair choice to join a union. The law guarantees that choice.”

Leading up to the Bessemer vote, The Times did a reporting deep dive on a similar effort at an Amazon warehouse in Chester, Va., in 2014 and 2015, where a union tried to organize about 30 facilities technicians.

The reporting offers one of the fullest pictures of what encourages Amazon workers to open the door to a union — and what techniques the company uses to slam the door and nail it shut. Some of those tactics have been used in Alabama, too.

“Where will your dues go?” Amazon asked in a notice posted in a bathroom stall, which circulated on social media. Another proclaimed: “Unions can’t. We can.” Amazon also set up a website to tell workers that they would have to skip dinner and school supplies to pay their union dues.

Bill Hough Jr., a machinist at the Chester warehouse, was one of the leaders of the union drive. Amazon fired him in 2016. In an interview, he suggested that Amazon’s customers just don’t know how miserable a job there can be.