Ronald Reagan, exploding the theory that conservatives are gloomy by nature, has proposed a 1983 budget which, in wishful thinking, rivals Lyndon Johnson's romance with guns and butter.

The president is obstinately clinging to the hope that deficits created by radical tax-cutting will generate, not huge inflation and high interest rates, but substantial economic growth.

Not that Reagan's underlying impulses are wrong. He has said repeatedly that the nation needs fiscal retrenchment, balanced budgets, renewed capital growth, and a stable currency. He is right. The program is so sound that he ought to give it a try.

Operationally speaking, Reagan has not tried it. He has put his imprimatur on the largest peacetime deficit in history and dispatched David Stockman to explain its advantages to Congress. Whether Congress will buy used projections from this man is another question.

By every index except the containment of inflation (and 8 to 10 percent unemployment will usually contain inflation to a degree and for a time), all the vital signs are unstable. Wall Street's advance valentine for the President, on budget day, was a 17-point selloff of stocks and another big plunge in bond prices. The financial markets anticipate trouble.

Meanwhile, the big boom in private-capital investment that was to be an important dividend of supply-side economics has taken an odd turn. Private investors, their cash balances swollen by tax cuts, are rudely disregarding the administration's theory and pouring their funds into the money market.

The Democrats, of course, are gleeful. They see themselves as potential political beneficiaries of an economic plan that shows Reagan to have the courage of their most daring convictions. But their most visible spokesman, Speaker Tip O'Neill, is as fuddy-duddy in his way as is his sparring partner, the president. The liberal Democrats have no ready formula for reversing the slide to national insolvency.

The significant initiative, in fact, may be coming from another quarter. As Washington was fussing about budget figures, Missouri became, last week, the 32nd state to call for a "constitutional convention." The idea is to frame a "balanced budget" amendment that would hold federal spending to a fixed percentage of GNP.

This is obviously a rigid, unsatisfactory remedy for chronic fiscal irresponsibility. But it might force a return to the lost art of making real national choices, consistent with real national revenues.

>Solvency by constitutional fiat is a last resort, and a drastic one. But the idea is rapidly gaining adherents. If it materializes, it will be a monument to both the failure of rational legislating and the failure of conservative opportunity. It is ironic that Ronald Reagan should be hastening the day.

There are many forms and definitions of conservatism. But at the least, conservatism uses traditional forms and institutions prudently to head off radical remedies. That was the way of Reagan's most distinguished and effective modern predecessor, Dwight D. Eisenhower.

Eisenhower, a realist, did not waste his energy fighting a rear-guard skirmish against the welfare state. He bent himself to restrain defense spending, to balance the budget and to keep the dollar stable. He enjoyed some success at all three objectives, which were mutually compatible.

Ronald Reagan has failed so far to work out the internal contradictions in his program. He has also forgotten, or not yet learned, that effective conservatism begins with a sense of the possible.