• The Retail, Wholesale and Department Store Union on Thursday accused Amazon of illegally interfering in the union election at the company's Bessemer, Alabama, warehouse.
• The union filed 21 objections to the National Labor Relations Board, alleging Amazon fired a pro-union employee and threatened to close the warehouse if workers voted in a union.
• The election remains to close to call and hinges on several hundred challenged ballots.

Amazon illegally interfered in a recent union election at an Alabama warehouse, according to a statement on Thursday from the Retail, Wholesale and Department Store Union the union, which organized the campaign.

The RWDSU on Thursday filed objections to the National Labor Relations Board, claiming Amazon "created an atmosphere of confusion, coercion and/or fear of reprisals and thus interfered with the employees' freedom of choice" to join or reject a union.

The complaint comes a week after the NLRB finished tallying ballots in a closely-watched election at Amazon's fulfillment center in Bessemer, Alabama. At the facility, known as BHM, union supporters narrowly trailed opponents, but 416 challenged ballots remain. Of the counted ballots, the anti-union side is only up by 118 votes.

The NLRB will set a hearing to review the union's objections.

The RWDSU is taking advantage of momentum in the labor movement within Amazon and more broadly. Last week, workers at an Amazon warehouse on New York's Staten Island overwhelmingly voted to form Amazon's first U.S. union, though the company is expected to file objections in the coming days.

And in Bessemer, the margin has tightened since last year, when workers held an initial vote on whether to unionize. In that election, which was conducted via mail ballot, the NLRB found illegal interference by Amazon.

Following the second election, the RWDSU filed 21 objections with the NLRB, accusing Amazon of threatening workers with closing the warehouse if they organized. The union claimed Amazon fired an employee who was an outspoken supporter of the union, and suspended another pro-union employee.

The RWDSU also accused Amazon of intimidating and surveilling BHM1 employees during the election.

"Amazon's behavior must not go unchallenged, and workers in Bessemer, Alabama must have their rights protected under the law," RWDSU President Stuart Appelbaum said in a statement.

Amazon spokesperson Kelly Nantel told CNBC in a statement: "We've said from the beginning that we want our employees' voices to be heard, and we hope the NLRB counts every valid vote."

The company also filed its own objections to the RWDSU's conduct, such as its communications with workers about an infamous mailbox near the warehouse. It also objected to the NLRB's decision to hold a mail ballot election, arguing it depressed voter turnout in the election.

The NLRB could order a third election at the Bessemer facility, depending on the evidence submitted by the RWDSU.

WATCH: Alabama Amazon workers vote down unionization in closer vote