Toyota Motor Corp. is expected to announce today that it will build a second auto assembly plant in the United States, a move that could give the Japanese automaker capacity to assemble 500,000 vehicles annually here by the end of 1993, according to company and industry sources.

The investment is expected to total $1.5 billion and be devoted to a new line of mid-size family cars to replace the aging Toyota Cressida. It also could help the company build mid-size and full-size pickup trucks in the United States, some of the sources said.

That would give Toyota the production capacity to become a more active competitor in two of the most lucrative segments of the U.S. car business -- the mid-size cars and trucks that are the bread and butter of General Motors Corp., Ford Motor Co. and Chrysler Corp., America's Big Three automakers.

Toyota has called a news conference for today in Georgetown, Ky., where it now builds cars and automobile engines on a 1,300-acre site and, according to sources familiar with the company's plans, will announce that the second plant also will be built at the Georgetown site. The sources said yesterday that Toyota is positioning itself to become a major U.S. automaker before the mid-1990s.

The company's program, the sources said, has three objectives.

First, Toyota wants to strengthen its position in the North American market, particularly in the United States, where it sold 892,261 cars and trucks in the first 10 months of 1990, a 13.7 percent increase over the 784,432 vehicles it sold in this country in the same period last year.

Toyota now occupies 8.4 percent of the U.S. auto market, but it wants to increase this percentage by building to an annual sales total of 1.5 million by 1995. It currently has the capacity to build 280,000 cars annually in Kentucky, where it now assembles the Toyota Camry.

Toyota also assembles vehicles with GM at a jointly owned plant, New United Motor Manufacturing Inc., in Fremont, Calif. The NUMMI plant has the capacity to build 240,000 vehicles annually, about half of which -- Geo Prizm subcompact cars -- go to GM. More than two-thirds of Toyota's U.S. sales currently are imports, however, which makes the company vulnerable to anti-import sentiment that occasionally sweeps Congress.

Toyota, following a manufacturing and political strategy first implemented by Honda Motor Co., wants to be seen as an "American company," sources said. That means the cars and trucks Toyota builds here also will contain 75 percent or more U.S.-made parts, making them domestic products and insulating Toyota from trade-restrictive laws.

Second, Toyota wants to be in place to use the United States as a base to export cars to Europe, thus avoiding any trade barriers that a soon-to-be-unified European Community might erect against Japanese products, sources said. Toyota-made cars in this country would be considered "American."

Toyota is building plants in Europe, but the company hopes to use a combination of imports and products built in the host country to increase its market share in Europe the same way it has built its presence in the United States, sources said.

Finally, Toyota could use some of its U.S.-made products to fill certain product niches in Japan, again following the lead of Honda, which ships home some U.S.-made Accords.