Is there a silver lining for US deficit in foreign-policy fiasco?

IN a crazy-quilt way, the fiasco of CIA mining of Nicaraguan harbors may end in helping President Reagan - against his will - with his deficit problem. Even Tinkertoy cartoonist Rube Goldberg might have had trouble diagramming this unexpected side effect. The cartoon panels would have to show something like this:

CIA-bought minelayer drops an explosive into meeting room of Senate Select Committee on Intelligence. Explosion sends conservative Republicans and conservative Democrats who have backed President's Central America policy - and his Pentagon military buildup in general - into orbit. Orbiting senators reenter Senate atmosphere in time for budget vote. Senate majority votes for a chopped-back defense spending - as it OKs other budget trims and a small bundle of tax increases. That vote sets the pattern for smaller defense increases in future budgets.

This is a down-payment year. Both President and Congress are promising restive voters that Washington's token cuts and token new taxes are a down payment on giant tax reform and deficit-closing that will start next January.

Defense growth may be undergoing a similar down payment on the future. The cut in Pentagon budget growth from 13 percent sought by President Reagan to the 5 percent that is likely to emerge from Senate-House compromise may be an augury for the future.

In this election year, voter attitudes and public-opinion polls are having an impact. Congress is hearing from constituents that the size of projected deficits is high on their list of concerns. And polls indicate the public is satisfied that Mr. Reagan has already accelerated the Pentagon vehicle sufficiently. When he received his mandate in late 1980, 3 out of 4 Americans polled agreed that defense spending needed to be sharply increased. A 1984 Roper poll shows that that belief is now held by only 1 of every 2 Americans, and is soon likely to be a minority position.

Reagan has an unusually consistent record of turning obstacles to advantage. Congressional budget cuts - and two other key factors - are likely to cut the deficit from a runaway $250 billion-$300 billion in the late 1980s to red ink of voter-disturbing issue softened before the election. Federal Reserve Board chairman Paul A. Volcker - as well as the electorate - may respond favorably if the red ink subsides.

Those two other factors are important.

One is the extraordinary way in which new business enterprises are creating jobs - and tax revenues. Over the past 20 years the big Fortune 1,000 companies have created a net zero of new jobs. But during the same two decades, smaller, more innovative, faster-growing, capital-generating newer companies have produced about 25 million jobs.

Without this yeasty growth, the American economy could not have absorbed the baby boom, the flood of women entering the job market, and job losses in other industries without heavier unemployment at both the top and trough of the business cycle.

Such small firms produced some 5 million new jobs in the past 18 months - admittedly an up period in the business cycle. And they and their employees are adding about $50 billion a year to federal tax revenues.

The second factor is revenue surpluses among the states. Data Resources Inc., a respected economic computer modeling firm, forecasts a total budget surplus for state and local governments of $64 billion by the end of this year.

Many members of Congress say this will provide an irresistible temptation to trim federal aid programs to those state and local governments. Even if it doesn't, those surpluses must be invested. They will therefore offset a sizable amount of federal borrowing.

A look at federal deficit figures over the past year indicates a downward trend in the most unbalanced budget since World War II. A year ago a $250 billion deficit was forecast for fiscal 1984. At the time of the so-called midcourse correction last August, that forecast dropped to between $215 billion and $220 billion, but the actual figure ran closer to $195 billion. During the past five months, the deficit has been running at an annual rate of $157 billion.

Congressional budget cuts, and added revenues from new companies and the states, should drop that debit figure still further as the US moves into its next fiscal year. But the battle should not be seen as over. Future deficits still loom. Congress has bitten only a few of the easier bullets it must chew on.

After next January's election, President Whoever and the 99th Congress must move quickly on tax reform and simplification.

That is a widely held view in Washington. How to get there is not. But many pragmatic economists who look ahead to the next several decades see a need for changes in at least three areas. They believe the maturing baby boom will increase productivity and growth through the end of the century. But then the elderly baby boom will come to rest on top of a much smaller work force - and a smaller taxpayer base. To deal with this inverted pyramid, the economists urge:

1. More savings incentives in the federal tax form. This might be accomplished by faster expansion of the IRA exemption from taxation. Or it might allow credits on the income from $5,000 or more in ordinary savings investment.

2. Another great bite-the-bullet revision of the social security retirement age, benefit-indexing, or both, along with medicare fee allowances.

3. Simplification of the tax structure and tax forms. The flat-rate tax may not be politically salable. But a flatter tax certainly should be.