Imagine a situation in which the world's foremost superpower would have to close many of its consulates and even some embassies and operate those that remain with skeleton crews lacking the language skills and funds to perform many of the routine tasks of diplomacy.

According to Ronald I. Spiers, the State Department's undersecretary for management, this vision of a diplomatic wasteland is likely to become a reality in the months just ahead if Congress persists in its determination to make deep cuts in President Reagan's $22.6 billion foreign affairs budget request for next fiscal year.

The threatened cuts are causing such concern in the department that Secretary of State George P. Shultz recently said he may have to "drop everything else" to rescue the funds from the congressional ax. Otherwise, Shultz said, the result will be "a tragedy for U.S. foreign policy" that would create an international perception of "the United States withdrawing from the world."

In an interview this week, Spiers said that Shultz's gloomy assessment doesn't begin to describe the havoc that the budget Congress contemplates -- between $17 billion and $18 billion -- would cause in the conduct of American diplomacy.

He noted that Shultz was talking primarily about whether the administration would have enough money to meet its worldwide foreign assistance commitments and to begin a long-range building program to make overseas diplomatic posts more secure from terrorist attacks. These "big-ticket items" -- $16 billion for foreign aid and $1.4 billion for embassy security -- account for the largest share of the administration's fiscal 1987 request.

Less obvious -- but of equal concern to Spiers -- is the effect that cuts would have on a part of the budget that he describes as "looking like nickel-and-dime stuff" that "tends to get lost" in the discussion about foreign aid and antiterrorist protection.

This is the $1.5 billion that the administration seeks to pay the salaries and expenses of the State Department's personnel and programs. In discussing the impact of major cuts in those funds, Spiers said, "The results would be so devastating that we would have to make revolutionary changes in traditional diplomatic practices."

Such changes, he said, "without question would have to involve some combination of wholesale closing of overseas posts, slashing our programs for language training, communications and computer modernization, freezing new hiring and entry into the Foreign Service and possibly even furloughing some of the personnel we already have."

Many members of Congress questioned whether such damage would be done, and suggested that the State Department is "crying wolf" in an attempt to evade its fair share of the administration-supported drive for spending austerity.

They noted that until recent weeks the department gave little priority to pleading its case on Capitol Hill. And they cited President Reagan's adamant refusal to engage in horse-trading with Congress by trimming defense spending and agreeing to devote more resources to domestic programs that are of more interest to the members' constituents than foreign policy.

Department officials said privately that Shultz believes Congress is holding the foreign affairs budget and other high-priority administration programs hostage in an attempt to force Reagan to agree to a tax increase.

But, Spiers said, these larger political considerations do not answer the question of whether the State Department would have enough money to do an adequate job of representing the United States diplomatically. While Spiers acknowledged that the budget outlook is grim "across the entire spectrum of foreign policy and national security considerations," he insisted that the impact would be greater on the State Department than on other federal agencies with responsibilities in that area.

That, he said, is because the department's tasks do not involve big weapons systems such as those of the Pentagon or complex technological and communications programs such as those of the Central Intelligence Agency or the U.S. Information Agency. It does not disburse large sums as the Agency for International Development does. As a result, Spiers said, it does not have the flexibility to cope with budget cuts by stretching out or deferring work.

"The State Department's job is to deal with other countries, to report and analyze the situation in those countries, to assist American citizens overseas and to provide services abroad for other agencies," he said. "Our only resource is our people, and 55 cents of every dollar that we spend goes to paying the salaries and travel duties of those people."

For the current fiscal year, Congress, after imposing the 4.3 percent cut mandated by the Gramm-Rudman-Hollings law, gave the State Department about $1.39 billion for these functions. For next fiscal year, Congress seems intent on freezing the department's salaries and expenses at roughly the same amount.

But Spiers estimated that after adjustments are made for foreign currency exchange rate losses and a pay raise passed by Congress, the department will be left with about $1.276 billion to pay for existing programs. That, he stressed, does not take into account plans for such important new projects as the opening of a consulate in Kiev.

When the costs of continuing and projected new programs are added, the department believes it will be $176 million short of the funds that it needs to operate next year.

To accommodate the fiscal 1986 Gramm-Rudman-Hollings cuts, the department ordered the closing of seven consulates and cut back sharply on hiring, travel and purchases of equipment. To absorb another big shortfall in its funds, Spiers said, it will have to extend these practices "to the point where they do damage that will take years to repair."

A departmental task force headed by Charles W. Bray, former ambassador to Senegal, has identified $129 million that could be saved, largely by closing 40 of America's 255 foreign missions. Most would be consulates, but in some areas, notably Africa, the Bray group's recommendations envision shuttering U.S. embassies and having one ambassador to cover four or five countries.

"Even that wouldn't give us the necessary savings," Spiers said, "and if it did, we still would be very reluctant to take a step like that. Although we've ranked every post in relative order of importance . . . you never know when changing circumstances will make it vital for us to be represented in a particular area, and we could find ourselves caught without anyone there."

The seven consulates now being closed include the one in Bremen, West Germany, which is the oldest outpost of American diplomacy. Spiers recalled that a delegation from Bremen told him that they were thinking of putting a plaque on the consulate that would say: "Opened by George Washington. Closed by Ronald Spiers.