A federal judge on Tuesday temporarily blocked the Trump administration from moving forward with a dramatic reorganization of the Department of Health and Human Services, finding that the mass firings and organizational changes were probably unlawful.

In an opinion accompanying the order, Judge Melissa R. DuBose of the U.S. District Court for the District of Rhode Island said that Health Secretary Robert F. Kennedy Jr.’s efforts to wipe out entire programs and reorient the agency’s priorities and work far exceeded his authority.

“The executive branch does not have the authority to order, organize or implement wholesale changes to the structure and function of the agencies created by Congress,” she wrote.

A coalition of 19 Democratic-led states and the District of Columbia had banded together in a lawsuit led by Letitia James, the New York attorney general, seeking to reverse Mr. Kennedy’s plan to cut 10,000 federal health workers after mass layoffs began in April. The lawsuit also challenged his reorganization of the sprawling department, which included paring down 28 federal divisions to 15.

The layoffs and restructuring effectively eliminated programs that assisted local officials with a variety of public health issues ranging from testing for sexually transmitted diseases to anti-smoking campaigns to lead-poisoning outbreaks, the states contended, resulting in the loss of critical services almost overnight.

In her order, Judge DuBose, an appointee of President Joseph R. Biden Jr., laid out a withering civics lesson, offering the government “a brief reminder about the bedrock doctrine of separation of powers that governs the relationship between the United States.” She added that the changes at the agency would have knock-on effects for the entire country, and that the Constitution expressly prevented someone in Mr. Kennedy’s position from attempting such a ”concentration of power in one part of the government.”