If Japan's two biggest automakers finally decide to start building their popular models in American plants it will be against their better economic judgment.

Both Toyota and Nissan see American production as a kind of economic trap that would snap shut just about the time they would be expected to start rolling models off assembly lines there.

Their minds were not changed by the frontal assault launched here this week by United Auto Workers President Douglas Fraser, who prodded them with dire threats of protectionist legislation in the United States. If they do change their minds, it will be for political reasons, including their own government's gentle pleadings that something must be done to keep the American happy.

The argument that something has to be done quickly was reenforced this week when industry sources disclosed that Japanese auto sales in the United States took another big leap forward in January.

The figures show a total of 177,544 Japan-made cars sold there during the month, an increase of 86.5 percent over last January. Toyota's sales alone were nearly 120 percent greater than a year ago.

In interviews and public statements over the past week, both Nissan and Toyta defended that kind of record as the result of temporary phenomena not likely to be repeated and explained why it wouldn't make economic sense to invest in America now.

They claim their sales shot up only because American manufacturers failed to switch from the big gas-guzzlers to the small, fuel-efficient models in which Japanese firms specialize. When last year's new oil crunch came, the American consumer went for Japanese models as the only alternative.

Now, American companies are getting ready for an expensive retooling that will put fuel-savers on the market by the mid-1980s. It would mean the Japanese say, that if they decided now to open U.S. plants, the first models would be rolling off into a fiercely competitive market most likely to be dominated by General Motors and Ford.

The investment would be great and the risks too high, Nissan and Toyota executives say.