There is an inevitability to it, like death and taxes: eager new administrations of both parties come to town convinced that their economic policies will succeed where others' have failed.

The more or less standard forecast is for economic growth and price stability at the same time, plus--usually--a balanced budget in the final year of a first term. And when those rosy estimates wilt away, the newcomers explain failure with intellectual acrobatics that would make Olympic gymnasts proud.

The Reagan administration has been no exception. Consider, for example, Treasury Secretary Donald T. Regan, who told an interviewer the other day the reason the deficit is so high is that the administration has done so well in curbing inflation.

His point was that lower inflation means lower incomes and a lower tax take. He made no reference to other possible explanations for the deficit, the Reagan tax cut, for example, or defense spending increases. Nor did he mention that many see today's lessening inflation rate as the result, not of a positive Reagan anti-inflation program, but of a deepening recession that the administration didn't foresee.

As economist Ben E. Laden of the T. Rowe Price Associates, Inc. said the other day, "as events unfolded, it became clear that the administration had expected too much from its program, had oversold the results, had erred in its forecasts, and had probably made major tactical errors in implementation."

But as national production levels have fallen, factories have shut down and unemployment lines have lengthened in recent weeks, the president's men have devised ingenious explanations to explain why economic performance has not kept pace with earlier promises.

On the CBS "Morning News" Wednesday, when asked by correspondent Jane Quinn if President Reagan is not "embarrassed to be projecting these huge $100 billion and over deficits now," Regan chuckled: "This is one of the great faults of Washington. We've done so well that we're going to do poorly. Now, let me explain what I mean by that. We've done very well in getting the rate of inflation down. As you know, the consumer price index figures came out yesterday, which are clear indication that for this year, we will be in single digit inflation--the first time since 1978. But as inflation goes down, we in the Treasury, particularly the IRS, collect fewer taxes, because you're not taxing that inflated income. So the net of it is, our take will be less. So the better we do on inflation, the less our take will be, and that will cause more deficit. That's the sort of conundrum we're going to have to solve."

Quinn dismissed Regan with the "hope that you solve it." Not a word about the recession--definitely not in the administration's game plan at the start of the year--which threatens to push unemployment up to or past 9 percent.

No mention either of the administration's "supply-side" tax reductions which will total some $749 billion through fiscal 1986, nor the 7 percent real growth projected in each of the next several years in the defense budget, nor the impact of double-digit interest payments on the national debt.

Moments earlier, on the same show, Commerce Secretary Malcolm Baldrige had been interviewed by George Herman on the November CPI, which showed inflation subsiding to an annual rate of 6 percent.

Baldrige savored the "great news," and then volunteered: "That's the best kept economic secret in the press--the fact that inflation for this whole year is down 20, 25 percent from last year." Had it been up that much, Baldrige said, "I'll guarantee you, you would have seen it on the tube every night."

Herman asked if the low inflation rates in the past few months might not be an "aberration," a thought that Baldrige rejected. But he was not asked to explain why the inflation rate had come down, nor to discuss the connection between recession and lower inflation.

In the last few weeks, the administration's embarrassment over the mounting deficits has also propelled economic advisers Murray L. Weidenbaum and William E. Niskanen into public statements suggesting that deficits, in any event, are of little economic consequence.

Whatever the merit of that argument, advisers close to the president agree that it was a tactical error. This approach is not likely to be pursued because it collides with traditional GOP doctrine and Reagan's campaign statements.

But the Baldrige and Regan comments indicate that the administration is persevering. All recent administrations have had to find acceptable rationales for the economy's failure to live up to their promises. These explanations are one of this city's art forms. This administration is no exception.