The U.S. International Trade Commission, in a decision that could determine the affordability and availability of small foreign and domestic vehicles, will decide today whether the U.S. auto and massive layoffs were caused by the recession or by a deluge of imported cars and light trucks, particularly from Japan.

If the ITC finds that imports are the culprits, it will decide next Monday whether to recommend that the president impose quotas or other measures to limit imports, as the Ford Motor Co. and the United Auto Workers union have requested. If the commissioners find today that Detroit's problems were not caused by imports, as the foreign car manufacturers contend, only Congress can give the president authority to impose import restrictions, trade lawyers and ITC officials said.

The imported versus domestic car controversy and the indefinite layoffs of at least 220,000 autoworkers in major industrial areas were major campaign issues, and President Carter vowed to act quickly on the ITC's recommendation if the commission decides imports have caused injury. The ITC would formally send its recommendation by Nov. 24 to the president, who would have 60 days to act on it.

The deadline would be Jan. 24, four days after Inauguration Day, so theoretically President-elect Ronald Reagan could decide the matter. But administration officials have said they expect the president to rule by mid-December if the ITC finds Detroit is the victim of imports.

In addition to imposing quotas, tariffs or both, the president could begin negotiations with the Japanese for an orderly marketing agreement on import restrictions.

Any decision by the president must take into account political and social consequences, such as the number of autoworkers and related firms receiving government adjustment assistance, the effectiveness to any industry efforts to adjust to import relief would have on consumers and the economic interest of the country. The president also must consider any economic and social costs to taxpayers, communities and workers and the geographic concentration of the imported products marketed here.

Both sides in the case were reluctant last week to predict an outcome, but some foreign car proponents said they hope the five commissioners' car-buying habits are an indication of how they'll vote: three own foreign cars and two own American cars, an ITC spokesman said.

The case was filed by the UAW under Section 201 of the Trade Act of 1974, which requires proof that imports are the substantial cause of injury or pose a threat of injury to the domestic automakers. The section, known as the "escape clause," is supposed to allow sufficient time for the industry to adjust to international competition. The UAW and Ford say they need a five-year curb on imports to proceed with $80 billion worth of retooling and modernizing that will allow them to make large quantities of small cars.