Republican and Democrats in Congress teamed up Friday to oppose what appears to be a Trump administration plan to cut the foreign aid budget.

The administration hasn't announced whether it plans to seek the cut. But the Office of Management and Budget instructed the State Department and the U.S. Agency for International Development last weekend to freeze unspent money in their budgets. That would be a first step in seeking cuts to funding already approved by Congress.

The amount involved could be as much as $4 billion and includes money appropriated for United Nations peacekeeping, development assistance, global health programs and military training.

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The top members of the House Foreign Affairs and Senate Foreign Relations committees sent a letter to the OMB seeking to head off such a move and threatening a response if the administration moves ahead with one. Their letter points out that Congress appropriates the money under the Constitution and the money is "essential" to U.S. global leadership and security.

"Slashing crucial diplomacy and development programming would be detrimental to our national security while also undermining Congress's intended use for these funds," said the letter, signed by Sens. James Risch, Republican from Idaho, and Robert Menendez, Democrat from New Jersey, and Reps. Eliot Engel, Democrat from New York, and Michael McCaul, Republican from Texas.

"It would be inappropriate for any administration, under any circumstance, to attempt to override Congress's most fundamental power," they wrote. "Such action would be precedent-setting and a direct affront to the separation of powers principle upon which our nation was built. "

Last Saturday, OMB directed that unspent money in 15 different State Department and USAID accounts be identified and frozen. Under federal rules, most funding approved by Congress must be spent within the budget year intended or returned to the Treasury unless specific provisions are made. The current budget year ends Sept. 30.

OMB gave a similar directive to State and USAID last year and then proposed that the unspent money be returned to Treasury in a move that was rejected by Congress with the support of Secretary of State Mike Pompeo. The State Department has not commented on this year's order other than to say it will comply with the directive and provide the information requested by the OMB.

Since taking office in 2017, the Trump administration has sought each year to slash foreign affairs funding by as much as 30% in budget proposals that have been soundly rejected by lawmakers from both parties in Congress.