THE IDEA OF postponing the next income tax cut is gathering momentum in Congress. The first reduction in the income tax rates went into effect at the beginning of this month, and the next one is scheduled for July 1. Why not put it off, some senators ask, for three months?

Why not? Because it's several kinds of a bad move. Postponement might save $8 billion--not an insignificant amount, but small in relation to the requirements that are now looming ahead. The administration says that it needs $16 billion in either additional spending cuts or additional revenue to bring its deficit down to target by next September. A more realistic figure, based on a more realistic forecast of the economy, would be in the range of $35 billion. To judge from the comments of the Senate Republicans, let alone the House Democrats, it's unlikely that more than about $5 billion can be extracted from spending in 1982. That's why it's now urgent to find more revenue.

There's also a question of basic fairness. By next summer, the shift in the pattern of taxation will be apparent even to people who do not generally pay much attention to it. The corporate income tax has been very nearly abolished. The same thing is true of the inheritance tax. The top tax rate is down from 70 percent to 50 percent, with corresponding reductions in the capital gains tax. The postponed cuts would be those that would have mainly benefited taxpayers in the middle-income range. They might indeed resent that. It would be astonishing if they did not.

Congress needs to start thinking seriously about the broad tax increase that is becoming inescapable, if deficits are not to fly altogether out of control. Ideally, it is the personal income tax--the broadest and most equitable of all American taxes--that ought to be raised. But neither the administration nor Congress is likely to turn around so fast. Second best would be stiff excise taxes on natural gas, oil and gasoline. They could raise money in the amounts now necessary, and at the same time give the country another useful push toward energy conservation.

The talk about postponing that July rate cut for three months suggests that much of Congress still thinks that it can close the revenue gap by minor fixing, fiddling and procrastination. The gap is far beyond that kind of remedy, and it is getting steadily wider.