WASHINGTON — Days after warehouse workers on Staten Island defied Amazon and successfully formed a labor union, President Biden on Wednesday threw his support behind the workers and championed their cause.

In remarks to a national conference of unionized trade workers, Mr. Biden spoke directly to one of the world’s most powerful companies and defended the right of employees to unionize. “The choice to join a union belongs to workers alone,” he said during remarks at the national conference of North America’s Building Trades Unions. “By the way, Amazon, here we come. Watch.”

Jen Psaki, the White House press secretary, said the president was merely expressing his longstanding support for collective bargaining and unions.

“What he was not doing is sending a message that he or the U.S. government would be directly involved in any of these efforts or take any direct action,” Ms. Psaki said.

Still, the remarks were the most explicit about Amazon from Mr. Biden, who has called himself the “most pro-union president” ever and has long hinted that he disapproves of the company’s efforts to attempt to dissuade its workers from unionizing. Last year, Mr. Biden expressed his support for warehouse workers trying to unionize an Amazon warehouse in Alabama. But at that time, the president did not call out the company by name.

“Let me be really clear: It’s not up to me to decide whether anyone should join a union,” he said at the time in a direct-to-camera address posted on the White House Twitter page, after a pressure campaign by pro-union groups pushed him to weigh in on the drive. “But let me be even more clear: It’s not up to an employer to decide that either.” Workers there narrowly voted against forming a union. Amazon has also said that workers have a right to decide to unionize, but the National Labor Relations Board has filed a number of cases saying the company has improperly interfered with their right to do so. Amazon denies that.

The success of the unionization drive at the Staten Island warehouse — the only Amazon fulfillment center in New York City — caught many by surprise. Employees cast 2,654 votes to be represented by Amazon Labor Union and 2,131 against, according to the National Labor Relations Board, giving the union a win by more than 10 percentage points.

The victory comes at a perilous moment for the labor movement. Despite rising public approval for labor unions, high demand for workers and pockets of successful labor activity, the portion of U.S. workers in unions dropped last year to 10.3 percent, the lowest rate in decades.

Critics — including some labor officials — say that traditional unions have failed to devote enough resources into organizing campaigns and that they have often bet on the wrong fights.

Amazon is expected to aggressively contest the union’s win. An unsigned statement on its corporate blog said, “We’re disappointed with the outcome of the election in Staten Island because we believe having a direct relationship with the company is best for our employees.”

Amazon went on a hiring spree during the pandemic, which gave employees a growing sense of power while fueling worries about workplace safety. It now has 1.6 million employees globally but has been plagued by high turnover. T, was the subject of A New York Times investigation last year on the Staten Island warehouse, known as JFK8, revealed how many of its problems — including inadvertent firings and sky-high attrition — were emblematic of Amazon’s employment model more broadly.

The National Labor Relations Board has been pursuing cases in administrative and federal court where it says Amazon violated worker’s organizing rights. Amazon’s main response to the union win was that it believes the agency has lost its objectivity and was actively supporting the union, rather than being a neutral arbiter.

But the agency said its actions against Amazon were consistent with its Congressional mandate to enforce labor rights.

Katie Rogers reported from Washington, and Karen Weise from Seattle. Noam Scheiber contributed from Chicago.