The one-sentence email from the White House arrived in Cathy Harris’s inbox at 10:49 p.m. Monday, startling her when she woke up and read it early the next morning.

“On behalf of President Donald J. Trump, I am writing to inform you that your position on the Merit Systems Protection Board is terminated, effective immediately,” wrote Trent Morse, the deputy director of the presidential personnel office.

Ms. Harris, a Democratic member of the little-known but critical panel that adjudicates federal employee discipline, was the latest official caught up in the Trump administration’s effort to drastically reduce the size of the government work force and reshape it with loyalists.

In his assault on the federal bureaucracy, Mr. Trump has engaged in a multilevel strategy, targeting political appointees like Ms. Harris while instructing agencies to initiate plans for “large scale” reductions in staffing. Trump appointees, many working with Elon Musk’s downsizing initiative, moved quickly to place thousands of federal employees on administrative leave at the American aid agency known as U.S.A.I.D., as well as at the Environmental Protection Agency, the Education Department and the Department of Veterans Affairs.

But Mr. Trump’s blitz has run into a complicated set of legal roadblocks. In targeting executive branch officials whose positions are supposed to be protected by different restrictions against arbitrary removal, Mr. Trump has defied those legal limits. In a number of cases, the courts have already delivered setbacks to his efforts.

“Many of these systems have developed over decades, and they’re not necessarily designed at one point in time to achieve a particular purpose,” Don Moynihan, a public policy professor at the University of Michigan, said of the different laws protecting federal employees that Mr. Trump has targeted all at once.