Now the Reagan administration itself is looking for an alternative to Reaganomics.

The result may be nothing more than a new phrase, or a bit of high-powered PR to go with next year's budget, touting a second phase labeled "Reaganomics II." But meanwhile, the search is on--for exactly what isn't very clear.

Although pure supply-siders such as Paul Craig Roberts--an assistant secretary of the Treasury who quit the administration in February--will be the last to admit it, the huge tax cut, centerpiece of Reaganomics, failed to produce the boom Ronald Reagan promised.

And monetarism--the incompatible companion piece to supply-side economics--produced a recession. The administration's own internal forecasts see nothing better than a weak recovery from the recession, followed by a flat economy in 1983.

So in somewhat helter-skelter fashion, the administration is desperately trying to find a way out of the economic mess.

Reagan has already backed far off supply-side principles by agreeing to a new 1983 budget, pushed through by establishment Republicans on the Hill, calling for a $21 billion increase in taxes (many of them hitting upper-bracket individuals and corporations). For the next three fiscal years, the president endorsed $95 billion in new taxes, much of which represents a major retreat from the supply-side follies of 1981.

This should be credited to a gutsy performance by Senate Republicans, especially Sen. Robert Dole, chairman of the Finance Committee. He will now have to fight off entrenched business lobbies in what promises to be a bitter floor fight.

Some of the advance publicity on the administration's policy review has given misplaced emphasis to the notion that Reagan might try to curb the independence of the Federal Reserve. Such a curb on the Fed has only down-the-ranks support.

There are lots of other possibilities, so far very murky, including a "flat tax," which Reagan this week called "very tempting." In a pure form, this would do away with all loopholes and tax shelters, and provide a single low rate for everyone, including the very rich. But no one in this politically oriented town believes that a pure flat tax could pass Congress.

Many tax plans that are far from a true "flat tax" will get a public airing. According to The Wall Street Journal, Lawrence Kudlow, chief economist of the Office of Management and Budget, suggested to boss David Stockman "a fairness/equity/soak-the-rich marketing campaign." It would do away with a number of popular deductions while "moving in the direction of a simpler and more equitable tax code, with a tip of the hat to the flat-tax crowd."

A Wall Street Journal editorial observed: "This is not exactly the voice of a man who thrills to his President's program."

Roberts and the Journal's editor don't admit that the combination of supply-side economics and monetarism have raised hell with the U.S. economy, and indirectly with the world economy.

Other conservatives are more realistic. Treasury Secretary Donald Regan at least recognizes the need for contingency planning. At a Washington Post lunch the other day, Regan said: "If interest rates don't come down rather quickly, (if) unemployment hangs high, the obvious course of action (to) be demanded of us by Congress would be, do something, don't just stand there. . . . We have to consider what our actions might be. . . . You cannot wait until someone says-- you know, we're in a crisis, let's change. And then you say, to what? You have to anticipate . . ."

Implicitly, that recognizes that Reagan may have to alter his original program even more. But the unknown element is the president himself. He spent the better part of a year rejecting the tax changes that he now has reluctantly endorsed. Other concessions will be necessary to avert economic chaos for the rest of the decade. And whether he will buy them is an open question.

Kevin P. Phillips, the conservative theorist, says in his new book, "Post-Conservative America" that "the real conceptual bond" among the various contradictory strains in Reaganomics "is not logic, but reverie--an attempted re-creation of what Frederick Lewis Allen called 'the Indian Summer of the Old Order.'"

Phillips argues that many of Reagan's policy thrusts will have a better chance if "a second- stage" appraisal applies less rigidity to both "supply-side and monetarist shibboleths." He's right, of course, and so is Regan.

What Roberts and other hard-core supply- siders fail to recognize is that neither the public nor politicians will wait a couple of more years for the promised trickle-down from supply-side tax cuts to show up, or for interest rates to come down.