The union organizing workers for a second vote at an Amazon warehouse in Bessemer, Ala., is asking a federal agency to order the e-commerce giant to remove a controversial mailbox.

The Retail, Wholesale and Department Store Union filed a request for review with the National Labor Relations Board, asking the agency to tell Amazon it must remove a U.S. Postal Service mailbox on the warehouse property. The mailbox is a key reason that workers were granted a second chance to vote on whether to form a union after employees overwhelmingly rejected the effort in a vote last year.

More than 6,000 workers at the Amazon warehouse in Bessemer are eligible to vote on the union issue this year, and it would be a blockbuster win for labor organizations if it is approved. But workers voted to reject the proposal by more than a 2-to-1 margin last year.

The NLRB called for a new election after it found that the e-commerce giant improperly interfered in the first election. An agency official called out Amazon’s decision to place an unmarked mailbox in front of the facility just after voting started, writing that the company “essentially highjacked the process and gave a strong impression that it controlled the process.”

The union had expressed concerns about the mailbox, saying that it could make workers think that Amazon had some role in collecting and counting ballots, therefore potentially influencing worker votes.

Amazon did not immediately respond to a request for comment. The company said last year that the mailbox was intended to make voting easy for workers. (Amazon founder Jeff Bezos owns The Washington Post.)

In its election rules for this year’s vote, the NLRB said that the Postal Service “has moved or will move the mailbox” away from the warehouse entrance. No one is allowed to put a tent or sign or similar items near the mailbox. But the union said this does not do enough to remedy the issue and called on the agency to make sure the mailbox is entirely removed from the property.

“The mailbox’s continued existence on Amazon’s property stands as a stark physical memorial of a tainted election,” the union wrote in an email to media. In its request for review, the union said that Amazon moved the mailbox only a few hundred feet and that it is still in view of surveillance cameras and employees in the parking lot.

Ballots will be mailed to workers on Feb. 4.