A coalition of business, consumer and agricultural groups that claims to represent more than 60 percent of the U.S. gross national product yesterday launched an attack on sections of the Senate trade bill that they said "will backfire on the United States," worsening rather than reducing the trade deficit.

The group, which calls itself "Pro Trade," asserted that the U.S. trade deficit has begun to level off, led by a rebound in exports. Robert W. Roberts of the National Foreign Trade Council said his organization predicts the 1988 deficit "will be in the region of $100 billion," down from $168 billion in 1986.

And William C. Lane of Caterpillar Inc., a major construction machinery producer and exporter, said, "We are beginning to see import competition -- although it's still tough -- beginning to lessen." He credited lower interest rates, a decline in the dollar, and aggressive negotiating on the part of U.S. Trade Representative Clayton Yeutter for the improved outlook.

But Roberts, Lane and others of the Pro Trade group, organized to defend open world markets, conceded that the more favorable trend is not yet significant enough to derail Congress' drive to pass a trade bill this year.

The coalition -- which includes about 70 organizations -- therefore focused on specific elements in the bill it identified as "killer amendments" that would harm the U.S. economy or violate international trade obligations. The group said that an especially objectionable feature is the revised Section 201 in the omnibus trade bill about to be debated on the Senate floor.

Other features to which the coalition objected include:Mandatory trade targets, such as those called for by amendments sponsored by Rep. Richard A. Gephardt (D-Mo.) and under consideration in the Senate. Sector-specific protectionist language, such as that dealing with textile and shoe quotas. Antidumping proposals inconsistent with obligations under the General Agreement on Tariffs and Trade (GATT), the international compact that regulates world commerce.

The Gephardt amendment to the House bill would require countries with "excessive" surpluses and accused of unfair trade practices (as unilaterally determined by the United States) to reduce the surpluses by fixed amounts each year or face retaliation.

The new language on Section 201 cases that the Pro Trade group finds objectionable would remove presidential discretion in cases involving temporary relief to industries injured or threatened by injury by "fairly traded" imports.

At a press conference yesterday, several spokesmen for the group urged the Senate to support an amendment to be introduced by Sen. Bob Packwood (R-Ore.) that would permit the president to consider whether import relief would be in "the national economic interest."

In addition to Lane and Roberts, the Pro Trade spokesmen yesterday included William Maxwell of the Computer and Business Equipment Manufacturers Association; John J. Farrell, president of the International Terminal Operating Co. Inc., representing stevedore interests, and Perry Walker of Riverbend International Inc. and of the Fresh Fruit and Vegetables Association, representing farm interests.

Joining in the critique of the Senate bill were the Consumers Union and the League of Women Voters, which are not members of Pro Trade.

Speaking for all, Maxwell stressed that they support parts of the trade bill, such as the authority to negotiate a new multilateral trade agreement, and provisions to establish new international rules to protect intellectual property rights or encourage exports.

"The diverse cross-section of interests that we represent supports sound trade legislation," Maxwell said. "The Senate is at a crossroads as it begins floor consideraion of trade legislation. In one direction, we have the opportunity to open new doors to international trade.

"In the other direction, we slam trade doors shut through such market controls as Gephardt-type mandatory trade targets and tighter restrictions" on fair trade imports.

Mark Silbergeld of Consumers Union, noting that his organization doesn't join coalitions that include commercial interests, said that the union felt compelled nonetheless to speak out on the proposed Section 201 language because "there is no such thing as a free lunch. When the president considers relief in the case of fairly traded goods, he has to take into account all interests. If his decision is based exclusively on special interests, other interests will have to pay. And that's not good for consumers."

The group contended that the Packwood amendment was "a more balanced" approach. While the amendment permits the president, as is the case now, to use his discretion in deciding whether the national interest outweighs an International Trade Commission recommendation for relief, it directs him -- if he rejects the ITC recommendation -- to come up with an alternative program.