Sometimes Washington conceals more than it reveals. Take the pitched debate over President Reagan's economic program. It has obscured the president's larger political purpose. America is getting older at both ends; the retirement population is swelling, and the "baby boom" generation is moving into the age of family responsibility. From this, Reagan is attempting to fashion a coalition with a mass middle-class constituency -- increasingly intent on stability and security -- at the center. And he may succeed.

Little of this emerges from today's verbal and political crossfire. On the one hand, the administration portrays its program as the tastiest thing since corn flakes; it says that by 1984 inflation will be down below 6 percent, and unemployment will have dropped to 6.4 percent from 7.3 percent today. On the other hand, Democrats ridicule many of Reagan's ideas as economic moonshine.

But the political essentials of Reagan economics aren't difficult to glean. Of voters with incomes between $15,000 and $25,000 (median family income in 1979 was $19,700), 53 percent voted for him; of those with incomes between $25,000 and $50,000, 58 percent did; of those with incomes over $50,000, 65 percent did. His program attempts to satisfy their needs and wants.

Tight money aims at reducing inflation on the assumption that most people are working and worry more about accelerating prices than the threat of unemployment. Tax cuts are proportional, thereby giving relief to the middle classes that pay most of the taxes. Spending cuts fall heavily (though not exclusively) on the poor, the cities and the lower middle class, where Reagan's support is meager anyway.

There is even an unwritten Reagan "industrial policy" appealing to well-paid blue-collar workers. Spending increases concentrate on defense industries; liberalized depreciation aims to spur investment in heavy industry.

It may be true, as critics (and some administration economists) say, that tight money prevents the economy from growing as fast as official projections. But this may not be as damaging as is commonly supposed.

Consider this recent study from the congressional Joint Economic Committee.

Having put the Reagan program through a computer model of the economy, committee chairman Henry S. Reuss (D-Wis.) triumphantly concluded that the White House proposals would "by 1984 . . . result in America's first budget deficit of more than $100 billion [with] little change in inflation unemployment and high interest rates."

Sounds like a Republican disaster. Read on a bit.

On inspection, it turns out that the unchanging measure of inflation cited by Reuss is a little-used and dubious concept called "core" inflation. By the two most common measures -- the consumer price index and the "deflator" of the gross national product -- the inflation rate drops to about 8 percent, 2 to 4 percentage points lower than today. Moreover, unemployment (just above 7 percent) won't be any worse.

As for the $100 deficit figure, two qualifications need to be made: First, given the inflation expected between now and 1984, much of the higher deficit (about half) simply reflects higher prices; and second, the federal tax burden, according to the mdel, would be substantially (about 10 percent, or $80 billion) lower than it is today.

The hunch here is that if the economy performs as the committee model predicts, it would be an asset for Reagan. People may want the best of all possible worlds, but most realize they can't have it. What they really demand is that things to in the right direction.

President Carter lost because, among other reasons, he gave the impression that events were slipping out of his control. Between 1977 and 1980, the unemployment rate (after an initial decline) was essentially unchanged, while inflation doubled. To succeed politically, Reagan needs more to move toward his goals than to reach them.

He has no guarantee that he will succeed, of course. Since 1976, the economy has generated an unusually large number of jobs (11 million in total) for any given amount of economic expansion; a change in the relationship between economic growth and jobs could cause unemployment to go much higher than expected. A repetition of last year's poor harvests would produce grain scarcities and, ultimately, higher retain meat prices. Labor and production bottlenecks in the defense could aggravate inflation.

But for every danger, there's a possibility that inflation will recede more rapidly than expected. Oil prices probably won't rise as rapidly as they did under Carter. A strong dollar (partially the result of high interest rates) makes imports less expensive and tends to hold down prices of competitive domestic goods. And any decline in mortgage rates feeds back into the consumer price index, which affects wages and benefit payments linked to it.

Reagan's political opponents always have tended to underrate him. There are shades of that in the almost gleeful debunking of the administration's optimistic economic projections and its cherished catechisms such as supply-side economics. The tacit assumption seems to be that if reality repudiates these ideas and projections, the Reagan approach will collapse of its own weight.

Don't be too sure. There is a lot of innocence and, perhaps, some wishful thinking in this appraisal. Americans are so imbued with the notion of "progress" and so wedded to the possibility of perfection that they disdain discussing anything less. In this case, the prospect that a policy of slow economic growth -- perhaps near-stagnation -- could possibly pay off politically. Somehow it seems reprehensible, which is why the Republicans deny they're doing it.

But stagnation at the edges is, by and large, prosperity in the middle. Reagan is aiming to achieve a state of mind as much as a set of statistics. Sooner or later, his adversaries may have to come to grips with that.