An influential member of Congress will propose legislation this week that would compel the Pentagon to slash spending on its back-office operations by around $25 billion a year as part of a major bureaucratic shake-up.

Rep. Mac Thornberry (R-Tex.), chair of the House Armed Services Committee, wants to force the Pentagon to eliminate several defense agencies and tens of thousands of administrative jobs by 2021, according to committee staffers. He has scheduled a Tuesday news conference to unveil his plan, followed by a committee hearing on it Wednesday.

Legislative aides said the purpose is to streamline the Pentagon’s bureaucracy, which has grown bigger than any other in the federal government, with vast armies of civilian employees and contractors assigned to process payrolls, handle logistics and maintain data.

Under the plan, the savings would be reinvested in military hardware and operations. Overall defense spending, which is projected to reach a record $716 billion next year, would not be curtailed.

While lawmakers have tried for decades to make the Defense Department’s bureaucracy slimmer and more efficient, Thornberry’s proposal is notable because he is a defense hawk. He has pushed for bigger defense budgets, saying that spending caps in recent years have hampered military preparedness and training.

His legislation will probably be met with resistance from lawmakers who are fiercely protective of military-related jobs in their congressional districts, as well as from parts of the Pentagon itself, which has a track record of defeating or slow-rolling restructuring proposals.

Thornberry’s plan targets a sector of the Pentagon known as the “fourth estate,” a nickname for defense agencies that provide administrative, purchasing or oversight support to the armed forces.

Combined, those agencies consume 15 to 20 percent of the defense budget and employ an estimated 200,000 civilians and 600,000 contractors. Among the biggest examples are the Defense Logistics Agency, the Defense Information Systems Agency and the Defense Finance and Accounting Service.

Under Thornberry’s legislation, the Pentagon would be required to cut overall spending on the fourth estate by 25 percent, or about $25 billion, annually. It would also have to eliminate several agencies — the Defense Technical Information Center, the Office of Economic Adjustment, the Test Resource Management Center, the Defense Human Resources Activity and the Washington Headquarters Services — and reassign those functions elsewhere in the department.

In scope and savings, Thornberry’s plan mirrors a proposal that was drafted, and ultimately buried, by the Pentagon itself.

In 2015, the Defense Business Board, a federal advisory panel of corporate executives, released a study that concluded the Defense Department could save $125 billion over five years by eliminating administrative waste in its business operations.

Commissioned by the Pentagon, the study focused on the fourth estate and the armed forces’ excessive reliance on high-priced contractors and outdated information technology. But after the project documented far more wasteful spending than expected, senior defense officials moved swiftly to kill it by discrediting the results amid fears that Congress would use the findings as an excuse to slash the defense ­budget.

Since then, the fiscal climate has changed. The Trump administration and the GOP-controlled Congress have been willing to boost defense spending at the expense of widening budget deficits.