This could be a historic moment for the American labor movement. Amazon workers in Staten Island, New York, voted to join a union.

NEW YORK — Workers voted Friday to unionize an Amazon Staten Island warehouse, a historic decision that marks the first successful U.S. effort at the e-commerce giant and a major victory for the domestic labor movement.

Amazon, the country’s second-largest private employer, has long fended off attempts to unionize workers at its warehouse — a highly prized target among traditional labor groups who have seen membership wane in recent years.

But a small, upstart independent union led by a former employee of the Staten Island warehouse mounted the first successful campaign to unionize Amazon workers, breaking many of the traditional organizing rules and relying on workers‘ momentum.

The vote could start a cascading effect on other Amazon warehouses in the country, labor experts say, encouraging others to consider unionizing. That could transform the way the e-commerce giant conducts business and prioritizes the treatment of workers.

The Amazon workers at the JFK8 warehouse on Staten Island will need to ratify a contract to become union members, the next step in an already lengthy process that former Amazon worker Chris Smalls began last year as leader of the Amazon Labor Union.

A separate union vote brought by the Retail, Wholesale and Department Store Union in Bessemer, Ala., was tallied Thursday. The union failed to secure the vote, but it was close enough that the number of contested ballots that are still pending could change the result. The final result won’t be decided for weeks or months.