Washington — The ''eccentric approach'' of President Reagan's original economic plan did not work, and gradually the White House is shifting toward the ''old rules of economics that do work.''
Thus Herbert Stein, former economic adviser to President Nixon, traces the evolution of Reaganomics during the tumultuous first years of Mr. Reagan's term.
''The most obvious lesson of the past two years,'' says Dr. Stein, now a senior fellow of the American Enterprise Institute (AEI), ''was not that the policy was wrong, but that the promises made for the policy - no recession, balanced budgets, rapid, long-term growth - were unrealistic.''
Revision of these promises by the White House, says Stein, has seen the ''economics of joy'' replaced by the ''economics of austerity'' - a change he finds ''prudent and realistic.''
Unemployment will be higher, economic growth slower, and deficits immensely larger than the administration originally forecast.
Only in the reduction of inflation has the President exceeded his goal, and this not in the painless way he expected.
Experts agree that inflation was wrung out of the economy chiefly by the iron hand of the Federal Reserve Board, at the cost of millions of Americans out of work in the worst recession since World War II.
The myth of ''painless'' disinflation, said Martin S. Feldstein shortly after he became President Reagan's chief economic adviser, has been ''decisively proved wrong.''
Stein ticks off several flaws in the reasoning behind the President's original economic package:
* ''Never,'' he says, ''was there reason to think that these policies would be painless.'' Cutting inflation from 12 to 4 percent could not be accomplished without job loss and recession.
* ''Never was there reason to expect that Congress would enact the whole package.'' Lawmakers in fact trimmed taxes by more than Mr. Reagan asked and cut spending by less than he wanted, adding to future budget deficits.
* Most economists in 1981, including Stein, doubted that the whole Reagan package - even if carried through - ''would bring about the promised results.''
For one thing, says William J. Fellner, resident scholar at AEI and a former adviser to President Ford, a 10 percent nominal economic growth rate - on which Reagan policies were based - includes a rate of inflation that can itself induce further inflation. (The long-term growth rate of the economy is only 3 to 4 percent a year, although economic growth coming out of recession can for a period be higher. Nominal growth measures how fast the economy expands in current dollars, uncorrected for inflation. Real growth is the nominal rate minus inflation.)
Also, says Stein, the supply-side theory underlying Reaganomics foresaw a much faster growth of savings - supposed to result from the tax cuts - than actually took place.
Having said all this, Stein and Fellner - as well as leading economists associated with Democratic administrations - welcome the greater realism of the latest White House economic projections.
''The new projections,'' says Stein, ''are a significant step in the direction of realism and internal consistency.''
Both Stein and Fellner say the administration may be risking future inflation - or the rise of inflationary expectations - by planning for the economy to grow at an 8 to 9 percent nominal rate yearly.
A less risky growth rate, in the view of these two advisers to Republican presidents, would be about 5 percent yearly.
Most experts, while applauding the extent to which President Reagan has shifted economic ground, claim that too little is being done to narrow the huge deficits that loom ahead.
The deficits spring from a widening gap between government spending (about 24 percent of the nation's economy) and government income (shrinking to 19 percent under present tax laws).
Experts, including Fed chairman Paul A. Volcker, call for tax increases to narrow the gap. But Reagan holds fast to twin elements of his original tax plans - a final, 10 percent tax cut this July and indexing of the income tax to prevent bracket creep, beginning in 1985.
These measures, while attractive to taxpayers, will reduce US Treasury revenues and consequently boost the budget deficit.