HARDLY ANYBODY but its authors believes that the local content bill would actually save jobs for American workers. The idea is to protect jobs in the auto factories by prohibiting the sale of foreign cars if their local content -- that is, American components and labor -- is below a certain level. How high a level? It rises with the number of cars that each company sells, the formula having been carefully calculated to force Toyota and Datsun out of the American market. It's doubly bad legislation. Not only is its purpose wrong in principle, but as a practical matter, its effect will be precisely the opposite of its sponsors' intention. It will preserve fewer jobs than it destroys.

It will destroy jobs in several ways. Removing two strong competitors from the market will result in higher prices for cars, and fewer sales. But that's the least of it.

Americans, including those in Congress, keep forgetting that the rules of the game changed in fundamental ways in the early 1970s, when the trading nations began to let their currencies float up and down. If the United States passes a law to cut off a flow of imports, the effect is to raise the exchange rate of the dollar -- which is already high. A higher dollar means fewer American exports, and cheaper foreign imports. The drop in sales abroad destroys jobs in American export industries, while the rise in imports destroys jobs in the domestic industries that compete with them here at home.

This misguided legislation is being pushed mainly by the United Auto Workers. As President Reagan's trade representative, Willliam E. Brock, correctly argues in a letter to congressmen, "Imports have not been the root cause of the industry's problems." Most of the unemployment in the automobile industry has been the consequence of a long, severe recession and the general drop in demand for cars. But, as Mr. Brock also points out, new jobs have opened up in American manufacturing in recent years and most of them are in the export industries. Legislation that reduces imports will also, indirectly but inevitably, reduce exports -- and the jobs that depend on them.

The local content bill is about to come to the floor of the House of Representatives. A vote for it is a vote for a low-growth economy. It is a vote for an attempt to shore up the less competitive industries at the expense of the most competitive. It's a vote for the kind of industrial policy that gives priority to the status quo with continuing high unemployment, inflation and stagnant incomes. It's an attempt to offer a friendly gesture to those thousands of unemployed auto workers -- but it's a gesture that won't do them much good, despite the extremely heavy costs to the country.