IT WAS a message of great good cheer that President Carter bore to Detroit yesterday. The essence of it was that there will probably be restrictions on automobile imports Japan before the end of the year. In Detroit, Mr. Carter got the kind of warm response that political leaders generally get, at first, when they commit large and serious mistakes. But import protection for the automobile manufacturers is going to be bad for the inflation rate, bad for consumers and -- because protection is addictive -- especially bad for the auto industry itself.

Last month the United Auto Workers took their demands for import restrictions to the U.S. International Trade Commission, the agency that judges claims of damage. The ITC had, in the past, a marked bias toward protection. But its investigatiion would normally take six months -- that is, until shortly after the election. Most of the automobile industry, not to mention most of the Senate, would rather have the report arrive at the White House for action shortly before the election. What the president promised in Detroit was only that he would ask the ITC to accelerate its decision. He said nothing more. He didn't have to.

If the ITC finds that imports have injured the American auto industry, it will recommend remedies. The most costly and dangerous possibilities are quotas and tariffs, which invite foreign retaliation. That's what the UAW has requested.

The argument by some of the companies, and by the union, is that the American manufacturers need time to adjust to a sudden swing in American consumers' tastes. They need temporary protection, the argument goes, to keep the Japanese from sealing their market while they turn around.

Should you accept that argument? In 1962 the American textile industry got protection from imports on a similar argument -- that it needed time to adjust to changing conditions. Today, 18 years later, conditions are still changing, the industry is still adjusting and the original protection, repeatedly extended, is now wider and deeper than ever. The American steel industry has been under one form of protection or another most of the time since 1968. At present, it is, if anything, less capable of meeting foreign competition than it was 12 years ago.

In an industry as highly concentrated as automobile manufacturing, the only real competitive pressure on the dominant company, General Motors, comes from abroad. Import restrictions are an open invitation to the American manufacturers to raise prices. Protection for the automobile industry is inconsistent with everything the Carter administration has been trying to do to hold down inflation and raise productivity. But in this campaign summer, it looks as though consistency is going to run last.