THE DEMOCRATIC senators' panic-stricken reversal on the tax cut is absurd. They have now committed themselves to produce a tax bill by September. The odds are still against the actual passage of a tax cut this fall. That's fortunate, for the atmosphere of the next several months is not going to be conducive to rational legislation. President Carter is right, and Secretary of the Treasury G. William Miller is right, in wanting it deferred until next year. And, being right, they might usefully stick with their decision this time.

As for those Senators, they were preaching the virtues of fiscal rigor all spring. Why this sudden violent change of heart? Nothing has happened in June that was not utterly predictable in April. But now that the Republican nomination is assured to Reagan, some Republicans have introduced the first stage of the tax program that Mr. Reagan has been advocating since the beginning of his campaign. Required to vote down the Reagan program, the Democrats anxiously tired to take out a little insurance, before going home for the July recess, by kicking over their own position. As an example of calculated policy, this spectacle brings to mind a flock of sheep in a thunderstorm.

There is a good and reasonable case to be made for a tax cut in 1981, to be effective from the first of the year. Under present law, taxes will go up steeply. Social Security taxes will rise. The oil windfall tax will bring in much more money, as more money is decontrolled. Inflation will continue to push people into higher tax brackets without making them richer. Those three effects together will amount to a tax increase of roughtly $40 billion in 1981. It would be desirable to offset most -- but not all -- of that increase with a tax cut.

Since the size and timing of the proper tax cut seem clear, why not go ahead and pass it now" Because there is no agreement whatever as to who, precisely, should get the benefits. How much for individuals, and how much for business? And just which businesses? Under one scheme that currently enjoys a wide, if unthinking, popularity in Congress, there would be little for manufacturing industry but a huge bonanza for commercial real estate. Is that really what the country needs?

If a bill is to be passed before the election, in a summer distracted by two summer conventions and a fall campaign, Congress will hardly have time to read the language on which it votes -- let alone find out what it really means. If passage is deferred until after the campaign, as Speaker O'Neill suggests, there is the even more dangerous prospect of tax legislation by a lame duck Congress.

The Republicans cry that a tax cut is urgent to stem the recession. As they know very well, the legislation will have no effect on the economy until next year in any case. A tax bill cannot affect the depth of the recession; it can only affect the course of the recovery that follows. Legislation is best left to next winter, when the recession's damage can be accurately assessed, and the requirements of the economy can be better measured.