SINCE MR. REAGAN'S economic strategy isn't working out, what should he--and the rest of the country--do? In his State of the Union address last week, the president declared that he has no intention of changing course. Next year's budget, to be published a week from Monday, will certainly reflect that decision. The opposition is still muted, possibly because no one seems to have any very happy alternative to offer. The Democrats deplore the high unemployment and the high interest rates, but deploring them is not quite the same thing as proposing a coherent remedy.

There are essentially two kinds of economic policy that have gained what you would call a certain intellectual respectability. One is to run the economy, deliberately, in low gear for a period of years. This would mean aiming for only enough growth to keep unemployment from rising and living standards from falling. The alternative would be to shoot for higher growth and invoke some sort of guidelines or controls on wages and prices to restrain the inflationary side-effects.

You will observe that neither of these possibilities is very attractive. As a practical matter, the second one probably has to be discarded altogether. There are very few people in American politics who think that any sort of guidelines, let alone controls, could be made to work effectively.

The low-growth path is not only possible but highly probable. It raises a different kind of danger. There is a deeply engrained American political tradition holding that a president ought to be able to produce high growth at will. Presidents repeatedly promise it and, when it fails to appear, both the president's program and the president himself suffer a severe erosion of public standing. That happened early in the Carter administration, to Mr. Carter's great cost, and it's happening to the Reagan administration now. Oddly, the damage is no less severe because most people thought the original plans pretty implausible in the first place.

Mr. Reagan, like Mr. Carter before him, has promised a future full of sunshine and real growth rates in the range of 4 to 5 percent a year. The highest rate consistent with the administration's inflation targets is probably somewhere around 2 percent a year. Supply-side hocus-pocus won't help. At 2 percent, unemployment would go down very slowly, if at all. That's not very inviting. But there's one thing to be said for it: it's better than the actual record of the past three years.

The present attachment to unrealistic rhetoric, on which administrations feel compelled to build unrealistic programs, is not merely unhelpful to the economy. It is positively damaging. The Reagan program is conspicuous for its internal inconsistencies, which inflict gratuitous strains and confusions on industries and on people looking for jobs. For example, the reason for the exceedingly high interest rates is not minor technical flaws in the Federal Reserve Board's operations, as the secretary of the treasury claims to believe. It is the gross inconsistency between the administration's desire to restrain inflation and its desire to cut taxes.

The American economy is resilient, tough and, despite all its current troubles, vastly prosperous. It is evidently in a period in which rapid leaps to further heights of prosperity are not going to be possible, for reasons that lie beyond even a president's reach. That is not a welcome idea at the White House, or anywhere else. But it's better to accept that fact, and work with it, than to keep publishing plans that are promptly exploded by events. Excessively optimistic policy certainly hinders economic performance. But the greater damage may well be done to the country's political leadership, as administrations repeatedly get themselves caught between actual experience and the expectations that they themselves have raised.