“Get rid of it. If we don’t eliminate it completely, we certainly need to cut its power and reach.” Donald J. Trump on the Department of Education on the

“The third agency of government I would, I would do away with, the Education, the ... Commerce and, let’s see ...” Rick Perry on the Department of Energy on the

“We are going to get rid of it in almost every form.” Donald J. Trump on the Environmental Protection Agency on the

Cabinet departments are sometimes combined or split, but rarely removed.

Over the last half-century, efforts to dismantle cabinet departments have been successful only once, when the Post Office Department was replaced with the United States Postal Service.

But presidential campaigns often call for the elimination of departments. Those pledges are typically meant to signal that candidates oppose the status quo, said George C. Edwards III, a political science professor at Texas A&M University. “When people actually get in there and find out what it does,” he said, “they realize there’s not a reason to get rid of it.”

The Education Department budget fluctuates, based partly on presidential politics.

Just a year after President Jimmy Carter made the Education Department a standalone agency, Ronald Reagan’s 1980 campaign platform included a plan to dismantle it. Once elected, President Reagan proposed moving the student loans and Pell grants to the Treasury Department and education civil rights enforcement to the Department of Justice. The idea was heavily opposed by teacher unions and received little support in Congress.

Mr. Trump has said that he would like to cut the “power and reach” of the department. His nomination of Betsy DeVos, who hasn’t called for its elimination, suggests a shift in mandate. She has pushed to privatize public education and could use some of the billions of the dollars currently sent to school districts to support a wider range of schools, including private, religious and online schools.

More than half of the current budget goes to student loans and Pell grants; Mr. Trump proposed a new income-based repayment plan for student loans during the campaign.

The Department of Energy’s budget is also often in the cross hairs.

President Reagan also tried to dismantle the Department of Energy, and he reduced the staff by more than 2,000 people at the start of his term. But his plan to shrink government control of energy markets and have the Commerce and Interior Departments assume more responsibility failed in Congress.

George W. Bush’s first nominee for energy secretary, Spencer Abraham, had co-sponsored the Department of Energy Abolishment Act in the Senate in 1999, but during his confirmation hearing in 2001, he said that he no longer supported the bill.

During his campaign for president, Rick Perry, Mr. Trump’s nominee for energy secretary, said he would do away with the department. Mr. Trump also said he would eliminate all federal spending on climate change, much of which is spent on technology development at Department of Energy laboratories. The department spends about two-thirds of its budget on nuclear weapons.

The Environmental Protection Agency’s goals may change, rather than its funding.

Since President Reagan slashed the E.P.A.’s budget in 1981, it has stayed relatively flat. The agency’s mission is to set and enforce environmental policy (rather than distribute federal money, as the Department of Education does). So partisan disputes tend to focus on how it does that, not on its funding level. Under President Reagan, the E.P.A. reassigned lawyers from the enforcement division and referred fewer cases to the Department of Justice; during President Obama’s tenure, the agency issued expansive regulations like the Clean Power Plan to limit greenhouse gas emissions.