The Reagan administration's chief trade official, William E. Brock, calls it "the worst threat to the international trading system to come before Congress in a decade." Sixteen major farm organizations have joined hands with big business in opposition.

The object of their attack is a fast-tracked, labor-sponsored bill that would restrict imports of cars by insisting that cars sold in this country have a high percentage of U.S.-made parts and be assembled by American workers.

The bill comes up before the House Rules Committee this morning with the strong backing of House Speaker Thomas P. (Tip) O'Neill, who believes it will send a signal to the Japanese that the American people no longer will stand for trade barriers to U.S.-made products. O'Neill promised outgoing United Auto Workers President Douglas A. Fraser that the bill would be brought to the floor in the lame duck session of Congress, where it is considered a sure thing for passage. The 222 congressional co-sponsors represent more than half the membership of the House.

There are indications, however, that the Rules Committee hearing might last beyond today, as the opponents trying delaying tactics, hoping the bill will be set aside in the press of other business during the hectic final days of the lame duck session.

"The more we push it back, the less chance it has," said one opposition lobbyist.

Business- and trade-oriented groups admit that they have gotten off to a slow start in fighting the bill. Despite the large number of co-sponsors, the bill was not given much chance of passage until it swept through the House Commerce Committee on Sept. 15 after only one sparse hearing.

It is still not given much chance of passage in the Senate.

Local content is being sold by labor as a jobs bill, with the UAW saying it will save or create 1 million American jobs in the auto field. The Congressional Budget Office, however, has estimated that the bill would end up costing 66,000 American jobs as foreign retaliation would eliminate far more jobs than the legislation would create.

"Passage of this legislation would be a disaster for American farmers," said Ed Andersen, master of the National Grange, which, along with 15 other farm organizations, entered into a natural alliance with the Automobile Importers of America to oppose local-content legislation.

"One-fourth of U.S. farm income is derived from export sales, including $15 billion annually to our major overseas customers, Japan and the European Community. They could react in many ways to limit U.S. agricultural exports," Andersen said.

The farmers were the first to join the auto importers, who estimated that 2,000 of the 7,000 imported car dealers in the United States might be closed if the bill is passed.

Big business will get into the act later in the week, with such export-oriented firms as Boeing Aircraft, Caterpillar Tractor and Xerox meeting for breakfast Wednesday at the U.S. Chamber of Commerce headquarters here. Also listed as opposed to the bill are trade associations for port authorities and the electronics, aerospace, computer, and semiconductor industries. They will be joined at the chamber breakfast by U.S. Trade Representative Brock and John C. Danforth (R-Mo.), chairman of the Senate Finance Committee's trade subcommittee.

Despite the array of farm, business and administration forces lined up against the bill, UAW officials called it "a very easy issue to lobby for these days."