While President Reagan celebrated his famous victory last week in persuading the House of Representatives to support more assistance to the contras battling the government of Nicaragua, the administration was setting itself up to lose a far more important struggle.

Sen. Lloyd M. Bentsen of Texas and Rep. Richard A. Gephardt of Missouri, two of the most influential mainstream Democrats in Congress, were writing the final details of a bill they plan to introduce before the July 4 recess. It would slap a 20 percent tariff on all imports from Japan, unless the Japanese show by declining trade surpluses that they have finally opened their home market to goods from abroad.

Bentsen is a senior member of the Senate Finance Committee and chairman of the Democratic working group on trade policy. Gephardt is chairman of the House Democratic caucus and a member of the committee that writes trade laws. The drastic move by two key legislators indicates the growing alarm in Congress about the flood of imports and the loss of jobs and markets for U.S. firms. Even more, it represents the rising frustration on Capitol Hill, in business and labor, about what many see as Reagan's almost willful blindness and indifference to this problem.

While Reagan spares no effort to put the squeeze on a pro-communist government in a tiny, impoverished Central American country, he is silent and languid in the face of monthly evidence of record trade deficits that have made the United States a major international debtor nation.

The rising flood of imports has stalled the growth of manufacturing employment in this country even during these relatively good times. Dozens of firms -- including the giants of basic industry and high-tech -- are moving facilities offshore, making permanent the loss of good American jobs.

But rather than leading what is clearly the survival struggle for America's future, Reagan prefers to throw his muscle around against the pygmy challengers in Nicaragua.

The last speech on trade policy was delivered almost nine months ago. A private-sector adviser to the administration on trade policy told me that, as far as he has been able to learn, Reagan has had only two full-scale briefings on trade policy in his time as president. The second, he was told, ended when Reagan recounted one of his stories about "driving down a California highway" while governor -- and seeing a "Buy American" bumper sticker on a Toyota. "That," he said, "ended the discussion."

Last spring, Reagan pulled Special Trade Representative Bill Brock out of his vital post, when he needed a new secretary of labor. Brock's successor, Clayton K. Yeutter, the former president of the Chicago Mercantile Exchange, was appointed on April 2, but his name was not actually sent to the Senate for confirmation until June 11.

The same tortoise pace applies across the board. John A. Young, the president of Hewlett-Packard Co., headed the President's Commission on Industrial Competitiveness, which submitted its report last Jan. 25. The heavyweight panel of businessmen, bankers, economists, lawyers and labor officials said, "Our ability to compete internationally faces unprecedented challenge from abroad."

When Young inquired recently when the president might have some response to the many recommendations the group had made in January, he was told by White House Chief of Staff Donald T. Regan that trade would not become "a serious agenda item" until the period between next September and February 1986.

Reagan made a lot of noise earlier this year about having a new round of multilateral trade negotiations. The May 15 confidential report of the administration's advisory committee for trade negotiations, signed by six leading businessmen and a top labor official, said that such negotiations "alone are incapable of dealing with the overall U.S. trade crisis," and "might even detract from . . . the desperate need . . . to develop a national trade policy."

But they are not going to happen. While Reagan was preoccupied with Bitburg and the unilateral U.S. embargo of Nicaragua, the French blocked the Bonn summit from setting a starting date for trade talks.

All this helps explain why two such moderate and influential voices on trade policy as Bentsen and Gephardt are preparing their high-tariff legislation against Japan. Both of them know it is a risky way to try to force open the Japanese home market.

"I'd forget all these things if we had a president who would make this (trade) a major priority for the country," Bentsen told me. But we do not. While Reagan fiddles in Nicaragua, the protectionist fires burn higher in Washington and around the world. There is a crisis here -- not in Central America -- and this president does not seem to know or care.