When officials from the Japanese Embassy held a small dinner recently for a visiting dignitary from Tokyo, they invited Susan C. Schwab, an assertive economist who is regarded as perhaps the most influential Capitol Hill aide on trade issues.

The Japanese harbor strong, conflicting feelings about Schwab, legislative director for Sen. John C. Danforth (R-Mo.). They are greatly impressed by her influence in shaping major trade legislation but piqued by her determination to clamp down hard on Japan and other feckless U.S. trade partners.

"Many people say that she is behind all these Japan-bashing things," said Yoshiji Nogami, economic counselor to the Japanese Embassy, who added that he doesn't agree. "Sen. Danforth is very, very strict on some aspects of trade issues . . . but I think she is one of the few who really is conversant with the actual trade issues."

Schwab, whose first job out of school was as an economist for then-special representative for trade negotiations Robert S. Strauss, has worked behind the scenes for Danforth for nearly six years trying to coax Congress to do what the Reagan administration has been reluctant to do: retaliate against Japan and other countries that refuse to engage in reciprocal trading arrangements.

Danforth, a leading advocate of tougher U.S. trade laws, is the past chairman of the Commerce Committee and the Finance subcommittee on international trade and represents a trade-sensitive state with large auto and shoe manufacturing industries. He has given Schwab wide latitude to pursue initiatives they agree upon.

Her critics, including some in the Reagan administraton, say Schwab is a closet protectionist whose get-tough notions could help trigger damaging international trade wars. M. Alan Woods, a deputy U.S. trade representative, said that while he agrees with many of Schwab's goals, "Our disagreements are disagreements of tactics."

But Schwab's admirers on Capitol Hill and in a small circle of international trade experts said she is a hard-nosed pragmatist, well versed in arcane trade economics, and a dazzling strategist and negotiator.

"She makes a good chess player in the trade game," said David Busby, a Washington lawyer who specializes in trade. " . . . . Her influence is all over the trade bill. And the interesting thing about her influence is that it seems as rampant now as when the Republicans were in power."

Schwab, a self-described "free-trade pragmatist," contends that the administration has mistakenly sought to "paper over" serious trade differences with Japan to preserve bilateral relations. If left unchanged, she said, that practice will lead to a "major blowout."

"The analogy I use with my Japanese friends is the earthquake," said Schwab, who spent nearly two years as a trade policy officer at the U.S. Embassy in Tokyo. "You are better off having periodic tremors, because if you don't have tremors, and your fault is there and it is static, when the big one comes it will do major damage."

Both Japan and the Reagan administration felt tremors of sorts this week, as the Senate approved its version of an omnibus trade bill to force the administration to take a more vigorous and systematic approach to fighting unfair trade practices and reducing the massive U.S. trade deficit, which last year approached $170 billion.

The keystone of the Senate bill is the "Super 301" amendment, an alternative to the controversial Gephardt amendment in the House bill that requires other countries to reduce their trade surpluses with the United States by 10 percent a year for four years or face automatic retaliation.

The Senate amendment, a variation of a proposal thought up by Danforth and Schwab last spring, would require the president to file trade complaints against nations that "maintain a consistent pattern of import barriers."

The measure, which was hammered out by Danforth and Sen. Donald W. Riegle Jr. (D-Mich.) with the blessing of Senate Majority Leader Robert C. Byrd (W.Va.) and Minority Leader Robert J. Dole (Kan.), would require the president to negotiate an end to such barriers by 1991 or impose some form of trade sanctions.

However, it would give the president wide discretion to determine the form of retaliation or to waive those requirements if he concluded retaliation would be against the national interest, an approach that is a far cry from the stringent retaliatory requirements of the amendment sponsored by Rep. Richard A. Gephardt (D-Mo.).

"The day you retaliate under 301, you've lost, because the object is to open foreign markets," Schwab said. "Gephardt is protectionist and is likely to lead to a contraction of markets."

The Senate amendment was stitched together by Schwab and other high-ranking staff members during a 72-hour marathon bargaining session that ran until 1 a.m. on July 10, the day the amendment was approved. Key advisers to Dole huddled in one room in the Capitol, Riegle and Byrd aides and labor officials powwowed in another, and Danforth and Schwab, who actually drafted the language, moved in and out.

"It came down to two sentences," Schwab recalled. "We spent two hours negotiating two sentences . . . . It was a fascinating exercise. I felt I was using everything I ever learned. You have nothing to offer up here but your credibility."

Hal Malmgren, an international business consultant and a former deputy U.S. trade representative, said Schwab excels as a strategic thinker and consensus builder. She is able to quickly synthesize the thinking of Congress, the administration and special-interest groups, he said, and also regularly consults experts in Tokyo, Brussels and other financial centers.

"She pretty much knows where everything is and who has what ax to grind," Malmgren said.

Schwab, the daughter of a retired Foreign Service officer, lived overseas for much of her childhood and then returned to the United States to attend college. "It was a fascinating way to grow up," she said.

In 1977, just out of graduate school at Stanford University, Schwab landed a job as an international economist in the office of the special U.S. representative for trade negotiations. She specialized in agricultural issues during the Tokyo Round of the multilateral trade negotiations.

Schwab served as trade policy officer at the U.S. Embassy in Tokyo in 1980 and 1981, where she learned much about Japanese culture, customs and trade practices, insights that would serve her well after she joined Danforth's staff as his trade expert.

"When I started out, the perception was that when the Japanese made a commitment in a trade negotiation, that commitment was credible," she said. "Over the last 10 years, that credibility is gone. The nature of the {Japanese} negotiators has changed, too. There is an arrogance in this generation that wasn't there in previous generations."

In 1981, Schwab's first year on Danforth's staff, the senator pushed for a plan to set quotas for Japanese automobile parts. That spring, Japan voluntarily limited its auto exports to the United States.

As the debate over the burgeoning U.S. trade deficit began to unfold, Danforth stepped up his criticism of the Japanese. In 1982, Schwab first drafted a bill for Danforth mandating reciprocity in trade with the Japanese and others, a concept that sprang from a brain-storming session that ultimately became the underpinning of Super 301.

The same year, sensing the vast trade implications of the breakup of American Telephone & Telegraph, Schwab drafted legislation that would force Japan to give up something in exchange for access to the wide open U.S. telecommunications market. That proposal, too, became part of this year's Senate trade package which was steered through by Sen. Lloyd Bentsen (D-Tex.), chairman of the Finance Committee.

Schwab is rated by many as among the most influential Hill staffers on trade -- certainly on a par with Jeffrey M. Lang, chief international trade counsel to the Finance Committee. Danforth credits Schwab with developing the Super 301 concept and telecommunications measure.

"I did not want to go the route of protectionism, but I didn't want our trade policy to be passive," he said. "Sue has envisioned how this works out and how it is put into practice."

"She has an incredible foresight and ability to look into the future of a bill and see what's going to happen to it," he said.