THE SERIOUS BATTLE over Reaganomics is just beginning. It has to do not with what the policy will be for the remainder of this year, which is pretty well determined, but with how the Reagan record will come to be regarded. The issue is not simply the substantive legacy but the lessons to be drawn.

The president's remarks in Cleveland this week are illustrative. They were only vaguely directed at Congress, with which his fiscal business is already winding down. The deficit will be the next administration's burden. Nor did the president seek to validate a particular policy as to interest rates and the dollar. Here, too, the goal for the balance of the term is defensive -- keep the currency strong enough to attract necessary foreign funds and to control inflation, but not so strong as to touch off an election-year recession. The president was looking beyond his own term.

He said: the budget deficit is not his fault, the trade deficit is not all bad, the stock market crash three months ago had to do with neither, and "the economic facts" don't point to a recession. If a recession occurs, he said, it will be because the media and "some of those doom-criers scare the people into one."

You've heard a lot of this before. Tax cuts didn't bring on the deficit because "every time we have reduced the rates, we have increased the total revenue paid" as the economy expands. The defense buildup didn't bring on the deficit either. Indeed, there hardly was a buildup: "in five years, the Congress has cut $125 billion from defense spending" while domestic spending has continued to rise. So goes the giddy revisionism, down to the final washing of hands: "when I hear them call it a stepchild of mine, I would like to point out something. The president of the United States can't spend a dime; only the Congress can do that."

The trade deficit, Mr. Reagan said, is also overdrawn. It is not another sign of overindulgence, as the critics gloomily suggest, but in some important ways a sign of strength; "the United States . . . has a trade deficit because our growing economy enables us to buy their goods." The stock market crash "was induced within the marketplace, and not from . . . some factor outside." "I don't believe that the dollar or anything outside of Wall Street and the markets had anything to do with the great debacle in October."

This administration has a lot that it can boast about. The tax code and most major spending programs, including Social Security, have been constructively reformed. Defense spending, which had been allowed to fall, has been restored. But in the aggregate, the administration has been mindlessly extravagant. The argument is sometimes made that the deficit has been useful as a discipline, deterring Congress from spending. That is true to a degree, but it is a strained defense.

The deficit is a terrible legacy, for which the country will be paying socially as well as financially for years. The only thing worse would be to believe the gloss now being put on it. That would condemn another generation to repeat what the country should repent instead.