THE DEMOCRATS running for the presidency in 1984 are fiercely tempted to go flat-out protectionist. Organized labor says that it is going to throw its unified support behind a chosen candidate early in the campaign. Among the candidates, the bidding is getting hot. It is depressing, but not particularly surprising, to see Walter F. Mondale out on the union convention circuit winning enthusiastic applause with speeches about the unfairness of letting Japanese companies sell all those cars here to people who would otherwise buy American.

The American anxiety over Japanese imports is turning into an obsession. The danger in that obsession is that it deflects people's attention from the real causes of economic distress and instead only generates xenophobia, the most unhelpful of public responses. In its most common form, the accusation against the Japanese is that they enjoy open access to the American market while they make it difficult for Americans to sell there. That's not entirely wrong. But it's hard to think that, with the most open market in the world, Japan would buy many American cars. The most expensive ingredient in a car is labor, and the Japanese factories produce a compact car with 45 percent less labor than the Americans do -- and with better quality control to boot. The American automobile manufacturers have serious disadvantages in world competition, but access to the Japanese market is the least of it.

In steel as in autos, the companies have lost control of wages; labor costs in both industries now run about twice the average for the American economy. Those extremely high labor costs leave producers vulnerable to foreign competition. But that's a subject you'll rarely hear mentioned at union conventions.

Despite some specific industries' loss of position, the American economy as a whole remains highly competitive on world markets. The United States exports far more in manufactured goods than it imports, and it exports far more in agricultural products than it imports. That's how it pays its oil bill.

Management and labor in several declining industries -- most notably steel -- are carrying on an aggressive campaign to persuade Americans that the whole economy is sinking. That is simply and flatly untrue. Taking the economy all together, American labor productivity is still the highest in the world, by a substantial margin. Protecting American steel and auto manufacturers by barring foreign imports does not save American jobs. It only redistributes unemployment, to the benefit of the protected producers at the expense of everyone else.

The structure of the American economy is now going through a period of deep historic change. Some sectors -- energy, communications, information -- are rising in wealth and competitive position. Others are losing. It's a painful process for American society, and particularly for the people who are caught in the shrinking industries. They are entitled to special consideration and aid. But neither they nor the country is helped by politicians who tell them that all their troubles are to be blamed on the Japanese -- and Mr. Mondale knows it.