Some 5,800 workers at the company's facility in Bessemer, 20 minutes southwest of Birmingham, have been sending in their ballots by mail since voting opened in February.
The vote ended Monday and the National Labor Relations Board has begun counting the votes.
The employees are seeking the right to collectively bargain over workplace conditions, wages, safety standards, breaks and other matters.
The decision could have a cascading effect on the company, which is the second-largest private employer in the U.S.
Agents from the labor board began counting the more than 5,800 worker ballots on Tuesday at 10 a.m. U.S. Central Time. The process is expected to last days and result in legal challenges.
The union drive has sparked the interest of politicians all along the political spectrum. Sen. Bernie Sanders (I-Vermont) has supported the worker drive and President Joe Biden has indicated that he backs them.
Earlier this month, Sen. Marco Rubio (R-Florida) said in an op-ed piece in USA Today that he backed the Bessemer workers.
“Here’s my standard: When the conflict is between working Americans and a company whose leadership has decided to wage culture war against working-class values, the choice is easy — I support the workers,” he said.
The Amazon workers have been vying to join the Retail, Wholesale and Department Store Union. If they succeed, they would be the first U.S. Amazon employees to unionize, Reuters reports.
Last week, about 50,000 workers at Amazon's Italy operations - including staff at its logistics arm and independent delivery workers -- reportedly went on a one-day strike to seek tighter rules on workloads and on driver shifts