Washington — Congress has watched growing fleets of Toyotas, Datsuns, and Mazdas on American roads, while auto workers jam American unemployment offices. And this week it is preparing to ''send a message'' that it is losing patience.
Bowing to intense pressure from labor unions, the House is debating a bill to require automakers who sell in the United States to use minimum percentages of American parts.
If enacted, the bill would halt a trend of American automakers buying parts abroad and force foreign car companies to build more plants here or cut their exports. By 1986, cars sold here would have to contain up to 90 percent ''domestic content.''
Almost no one on Capitol Hill gives the legislation a chance of final passage during the waning days of the lame-duck session. Although congressional leaders expect wide approval in the House, they say the bill has little chance in the Senate.
Even if it falls short this session, the attempt is another sign of the protectionist mood that has come over Capitol Hill since the elections. A week ago the House slipped in a ''buy America'' amendment to the 5-cent gas tax bill. That provision would require that the highway and mass transit funds be spent to buy only American made concrete, steel, and manufactured products.
''Everytime (there's) a depression or recession,'' says House Speaker Thomas P. O'Neill Jr (D) of Massachusetts, ''the House goes very protectionist. . . . Some of us have a strong feeling that a message should be sent to some of the governments.''
House majority leader Jim Wright (D) of Texas told a press breakfast last week that ''there's going to be more and more protectionist sentiment expressed.'' And Sen. Bob Dole (R) of Kansas has warned that the Senate Finance Committee, which he chairs, will spend the ''lion's share'' of time on trade issues.
Members of Congress complain that while Japan and others enjoy the US free-trade policies, they set up barriers to keep US products out of their own markets. Fanning their discontent, American trade negotiators came back empty-handed from talks in Geneva on reducing restraints.
''The President is making speeches about free trade, while the rest of the world increases restrictions,'' says Rep. Richard L. Ottinger (D) of New York, sponsor of the domestic-content bill. ''We need to send a clear message that if the administration doesn't act, Congress will.''
But the danger in passing protective laws is that they might touch off a trade war like the one that engulfed the world in the 1930s. Mr. Wright speaks of sending a ''shot across the bow of Japan'' as a warning. Opponents see that shot as akin to the sinking of the USS Maine, an event that sparked the Spanish-American War.
The Republican administration, switching places with Democrats who historically champion free trade, strenuously opposes the domestic-content bill. Secretary of State George P. Shultz warns that such a law might ''set in train a series of retaliatory measures by our trading partners.''
House GOP leader Robert H. Michel of Illinois writes in a dear-colleague letter that ''under the guise of reducing the job loss in one individual industry,'' the bill would create havoc in the economy and invite retaliation against farm products sold abroad.
''Our farmers would be particularly jeopardized at a time when they can least afford it,'' he says, citing the fact that 42 percent of American agriculture exports go to Europe and Japan and that Japan bought $6.6 billion in US farm products in 1981.
Foes also cite a Congressional Budget Office prediction that while the law might save 38,000 jobs in the auto industry, it would eliminate 104,000 jobs in other industries.
Groups ranging from the Chamber of Commerce to exporting firms and farmers are joining forces against the proposal.
Representative Ottinger concedes that there is a ''real danger'' of a trade war with legislation such as his domestic-content bill. ''I'm sincerely concerned about it,'' he says. But he adds, ''The question is what do you do? The Common Market and Japan are adding restraints (on trade) because of the hard times they're experiencing. How do you get them to take (us) seriously.''
''The US over a period of years has reduced barriers,'' he says. ''It made sense in order to build up Japan and Europe. But their economic strength has changed radically, and the trade relations haven't.''