IT HAPPENS every time a trade bill goes through Congress. For the following year or so, the country keeps stumbling across the damaging protectionist concessions that the administration made to get it passed. The latest discovery is particularly ominous because it constitutes a congressional attempt to manipulate the monthly trade statistics for an obvious and discreditable political purpose.

That purpose is to make trade deficits seem larger than they are, and trade surpluses seem smaller. That, in turn, is supposed to fan public fears of imports, to the benefit of those American industries -- shoes, steel, beef and the whole melancholy catalogue -- that are feeling the heat of world competition.

The clearest way to report the trade figures is to give the total value of both imports and exports without including the ocean freight and insurance charges; second best is to give the value of both with freight and insurance. The crooked way is to count freight and insurance charges only on the imports. Some years ago, the protectionists in Congress obtained a requirement that the Commerce Department calculate the trade balance that way every month, along with the traditional and straight way. As you might expect, nobody paid much attention to the crooked figures.

But the new trade act, taking effect in January, requires the Commerce Department to publish the crooked figures, which inflate the trade deficit about $1 billion a month, two days before the straight ones. The protectionist lobbies want that inflated number in the front-page headlines. Also, the crooked accounting will continue to show deficits next year after the trade balance has in fact turned into a surplus.

This assault on the numbers' integrity is not likely to have much practical effect. But modern economic policy is based on the statistical series that the government's computers churn out every month. Any attempt to distort these statistics, or manipulate the system that publishes them, has sinister implications. Congress has an obligation to repeal this dishonest restriction on the federal government's economic reporting.