On Monday morning of this week, President Carter crytically told his Cabinet, "I have made a decision on reorganization, and Vice President Mondale is going to implement it."

That was it. There was no explanation, no elaboration. No one asked him what his decision was.

For two full days the Cabinet members waited and wondered. Reorganization of government agencncies means gains or losses in personnel, funding, programs -- in short, it defines your empire.

Finally, on Wednesday morning, Mondale began making telephone calls to officials and congressional committee chairmen whose empires were involved. Carter made some, too. Their message was that the president had decided to try to create a new Department of Natural Resources but had turned down the idea of a new Department of Development Assistance.

In was a dramaic reversal for the Department of Housing and Urban Development and its secretary, Partricia Roberts Harris. Only a few weeks earlier the proposal to transform her department into DDA was a No. 1 option of the Presidential Reorganization Project at the Office of Management and Budget.

What intervened was the icy reality of politics. At one point, after a lengthy pulse-taking on Capitol Hill, White House aide Frank B. Moore told Carter, "Mr. President, it would cost you as much to get DDA as it's going to cost to get a SALT treaty approved."

Basically, White House and OMB aides found it would be almost impossible to overcome opposition to the idea of a super-HUD, which would absorb the Economic Development Administration from th Commerce Department and much of the Farmers Home Administration from the Agriculture Department.

Carter aides discovered the depth of the opposition when, over a 2 1/2-week period in late January and early February, they interviewed at least 60 senators and representatives and 50 leaders of various interests -- mayors, county executives, governors, labor, business and civil rights leaders. The most vocal opponents included smalltown mayors, rural interests and representatives from the South and the West. Their view was taht HUD is oriented too much toward the cities, especially big cities, and too much toward the minorities and the poor, the Northeast and the Midwest.

Another factor was the history of HUD. From its start in the late 1960s until a few years ago it was associated with bureaucratic red tape, urban renewal plans that failed and subsidized housing tht turned into abandoned, vandalized eyesores.

"Under Harris, HUD has cleaned up its act," said one White House aide. "Its manageent is vastly improved. That's way OMB wanted it to be the core of the new department. But there is an institutional memory in Congress, unfair as it is, and HUD is a victim of the memory."

Finally, EDA -- the Commerce Department agency that gives grants and loans to business and industry -- was a factor. When Congress passed its second local public works bill in 1977, EDA moved the money -- $4 billion -- in 90 days, and so far there has been no hint of scandal.

Not only is EDA considered efficient; it is becoming more urban-oriented under the direction of Commerce Secretary Juanita M. Kreps.

"Now the cities figure they have two friends in the administraton -- HUD and Commere," said one urban lobbyist.

With the Hill largely opposed and the interest groups indifferent or worse, White House political aides, according to one of them, felt that "we would lose everything if we tried for DDA." Oddly enough, Harris and her forces did no serious lobbying in Congress. "They felt they didn't need to -- they felt the merits of OMB's plan was on their side and would sooner or later be seen by everyone."

For her part, Kreps did not lobby much either -- except to tell anyone who would listen that DDA was a "lousy proposal."

Ultimately, under Carter's plan, Harris lost DDA but gained a request for an extra $275 million for HUD's Urban Development Action Grant program, whihc aids depressed cities.

Kreps wom the economic development battle because she not only kept EDA but won some new lending functions for it.

"A little something for everyone," said one White House aide. "It was the ideal decision-making process."