It could be an advertising agency's nightmare. Amid heated worldwide automobile competition and the annual hype surrounding new car models, two of the makers of America's most popular imported cars, Datsun and Toyota, are promoting the advantage of buying American.

They mention features such as the roominess of American small cars, their gas efficiency, style, low maintenance and affordability -- sales pitches usually reserved for their own models.

Unbelievable? But true. Toyota Morot Sales U.S.A. Inc, and Nissan Motor Corp. U.S.A. have resorted to praising their American competitors as part of their argument in the nation's largest and possibly most far-reaching trade dispute ever: that the recent surge in imported cars is not a threat to domestic automakers.

"The new American models are technoligically advanced, are highly fuel-efficient, have front-wheel-drive and are roomier than imports -- all important sales features to the American car buyer," Nissan said in briefs filed with the International Trade Commission. "The outlook for these models is especially favorable because they combine desirable features with American styling and will appeal to the American consumer's bias in favor of domestic cars."

Toyota claimes ". . . the foreign small car manufacturers will have a price disadvantage when competing with the new, attractive domestic cars. "Imported small cars have been priced substantially higher than domestic models in every year since 1972."

The ITC hearings next week will help determine whether import quotas and tariffs on imported cars should be established.Last June, the United Auto Workers, Union, with 300,000 of its members out of work, asked the government to set quotas to cut by more than half the number of foreign cars and trucks imported.

In addition, the union asked the ITC to raise duties on imported cars from 2.9 percent to 20 percent of the wholesale price. The union asked the government to maintain its 25 percent import duty on trucks.

The action, which would be effective for five years, would give the auto industry time to retool and produce its own gas-efficient small cars to compete with the imports, the union claimed.

The union's action is filed under Section 201 of the Trade Act of 1974, known as the escape clause, which provides temporary relief to industries injured by a substantial influx of imports. The purpose of the clause is to allow sufficient time for the industry to adjust to international competition.

The union must prove that it has been injured and that the injury was caused by a substantial number of imports, not just a few combined with mismanagement by U.S. automakers or other possible factors.

The Ford Motor Co. filed a similar petition in August.

The Japanese automakers, at whom most of the accusations are leveled, said in their briefs that the downturn in the economy and the fuel shortage are to blame for Detroit's sales slump. They said that imports pose no threat to American automakers because the U.S. economy is improving generally and more consumers will buy cars, most likely Americans cars.

They also said they were not exploiting Detroit's temporary inability to make small cars. Instead, they said, Detroit's problem is in selling large cars, which they say are not directly competitive with their small models.

Import restrictions would lead to a 10 percent increase in small car prices and reduce small car sales here by 9.6 percent, Nissan said. It also would raise the consumer price index by 0.5 percent and cost $4.5 billion to consumers in auto price increases, Nissan said.