Even before the start of the recovery, most professional economists began sounding warnings about the harmful effects of huge projected deficits. Although the projected deficits still remain about $200 billion a year as far as the eye can see, the economy has enjoyed nearly two years of vigorous recovery and is now advancing at a healthy and potentially sustainable pace.

Does all the good economic news mean that those early warnings were wrong? Is it possible that the projected deficits are not a problem? If the deficits have actually helped to drive the economy out of recession, won't they continue to stimulate growth? The answer to these questions is a simple, No, No, No.

The key to resolving the apparent puzzle is that the adverse economic effects of the deficit are building up slowly. The negative impact of a long string of deficits may not be immediately visible. But if there is not legislative action to reduce the projected deficits, they will have a serious cumulative effect on our economic future.

It's true that the government deficit helped to get the economy moving again in late 1982. That's why it would have been wrong to cancel or postpone the scheduled cuts in personal income tax rates. It was better for the government to pass up the potential tax revenue so that private individuals would have the ability to spend.

But while increasing consumer spending is helpful when the economy is in recession, increased national saving is essential for raising the long-term growth of the economy. That's the essential difference between "demand- side" economics and "supply-side" economics. The demand-side approach correctly asserts that a stimulus to spending can help to speed up economic recovery during a recession. But the supply-side approach is also correct in emphasizing that savings provide funds for investments that raise productivity and growth.

Trouble develops with the economy when savings are persistently diverted into financing a government deficit. Whenever the federal government runs a deficit, it has to make up the shortfall by borrowing in the financial markets. The money it borrows in this way would otherwise have been available for investment in business plants and equipment and housing. Although deficits start reducing the level of investment immediately, it may take several years before the cumulative effect of lower investment in depressing growth is generally recognized.

President Reagan, in his recent remarks to the annual meeting of the World Bank and the IMF, spoke of his desire for a period of increased world economic growth. He rightly pinned his hopes on the possibility of future gains in productivity. When there is an increase in the amount of output that the labor force can produce, that gain in productivity can be transformed into higher wages without inflationary consequences. Productivity gains can mean rising living standards for everyone.

But how can productivity be raised? The only reliable way is through increased investment. A worker with more equipment is likely to be a more productive worker. And the improvements in technology that the president referred to can be stimulated by a faster rate of investment in the newest types of equipment and by the availability of lower cost funds to finance corporate investment in research and development.

Investment in the United States is now being supported by an unprecedented inflow of foreign savings, at an annual rate of more than $100 billion, enough to offset more than half the current volume of government borrowing.

When the capital inflow from abroad begins to shrink -- and that will inevitably happen -- the full effect of the deficit on domestic investment will become more apparent. Without any foreign capital inflow, the rate of net investment in U.S. plant and equipment and housing would be one-third lower than it is now.

Although no one knows when the capital from abroad is going to dry up, the United States should not continue to live on borrowed time. To eliminate this risk and to achieve the productivity gains and rising standard of living that the president and all Americans want, the deficit must be reduced in each future year.

When President Reagan campaigned four years ago, many things were on his agenda: lower inflation, reduced tax rates, stronger defense, less government spending on domestic programs and a balanced budget. He has accomplished all of these but the last.

Those who question the president's determination to deal with the projected deficits in his second term should take note that in this current campaign the only promise on his economic agenda is to seek a balanced budget.