WASHINGTON — The State Department formally notified employees Thursday that it was about to begin layoffs as part of a consolidation plan that department officials say will reduce bureaucratic bloat but that critics call a shortsighted blow to American diplomacy.

In an internal message sent to State Department workers Thursday, Michael J. Rigas, the deputy secretary of state for management and resources, said the department would “soon” begin notifying U.S. employees who are losing their jobs.

Diplomats said senior department officials had told them to expect layoff notices as soon as Friday morning.

The layoffs are part of a reorganization plan unveiled in May by Secretary of State Marco Rubio, who called his department “bloated” and stifled by bureaucracy. Rubio said the changes would better align it with core American values and root out pockets of “radical political ideology.”

The State Department is proceeding with the cuts two days after the Supreme Court overturned a lower-court order that had blocked the Trump administration from implementing mass layoffs across the federal government.

The union that represents trained diplomats who rotate overseas, called foreign service workers, expects about 700 of those based in the United States to lose their jobs. A larger number of civil service workers, who work mostly in Washington, are also expected to be fired, in what is officially known as reduction-in-force actions.

In all, the department’s U.S.-based workforce of about 18,000 people will shrink by about 15%. Department officials said that more than half of that proportion would be made up of voluntary departures, including workers who have accepted the Trump administration’s “deferred resignation” offer.

Rubio’s plan does not include cuts to overseas staffing and operations, such as embassy closures, although a senior State Department official said that all the department’s operations worldwide were subject to ongoing review.

Democrats in Congress and veteran diplomats have decried Rubio’s plan, saying it will drain the government of diplomatic expertise at a time of global crises, including conflicts in the Middle East and Ukraine, and as the United States competes with China for influence abroad.

“America’s diplomatic corps is needed now more than ever to peacefully de-escalate ongoing tensions in the Middle East and achieve American foreign policy objectives in an increasingly complicated world,” several dozen House members wrote in an open letter to Rubio in late June. The downsizing plan, they warned, will “leave the U.S. with limited tools to engage as a leader on the world stage during this critical juncture.”

Critics also complain that the reorganization plan targets dozens of offices that deal with specific issues, such as human rights, democracy, refugees and war crimes.

Rubio contends that it is more efficient for regional bureaus to handle most of those issues, but veteran diplomats say they will inevitably be downgraded in policymaking.