President Reagan sought to dramatize his appeal for military aid to the Nicaraguan rebels yesterday by publicly inspecting a trove of captured weapons and listening to testimony about Sandinista "subversion" in Central America while a Republican-led congressional group hastily planned a two-day trip to Nicaragua and El Salvador.

The battle over helping the contras was waged yesterday with theatrics, political horsetrading, heated rhetoric and continued efforts on Capitol Hill to fashion a possible compromise if Reagan's $100 million aid request is defeated in the House next week.

Unhappy House Republicans from farm districts said they were using the issue to press the White House for more assistance to farmers.

Rep. Ron Marlenee (R-Mont.), one of a group of congressmen who met with Agriculture Secretary Richard E. Lyng yesterday, said, "It was not our intent to deliver an ultimatum. The only linkage is that we are being asked to support $100 million in contra aid and some of our producers are having a hell of a time out there."

The group also obtained meetings this week with Vice President Bush and other administration officials. On Wednesday Reagan announced a new $750 million farm credit package.

Members of the hastily organized traveling group, most supporting Reagan's proposal, said they will leave this morning. Rep. Robert K. Dornan (R-Calif.) said he looked forward to hearing Nicaraguan leader Daniel Ortega discuss what he said are Sandinista explanations for not negotiating with the rebels.

"We want to hear it to our faces one more time before we try to turn the debate" toward military aid for the rebels, he said. But another member of the delegation, Rep. Paul B. Henry (R-Mich.), said many church groups in his district are opposed to additional aid to the contras and he hoped to talk to church workers in the region. "I'm going down to learn, not to pontificate," he said. "I feel caught between the rhetorical cries on either extreme of the question."

A White House official said the tentative itinerary for the trip included a visit with one of the nine Sandinista "commandantes" and a visit to La Prensa, the only remaining opposition newspaper. The travelers will use a Defense Department plane.

In an interview with The Baltimore Sun published yesterday, Reagan said he might have to go back to Congress for additional funds if the Nicaraguan conflict is not resolved.

Reagan has been resisting compromise proposals that would delay or reduce his proposed $70 million in military aid and $30 million in non-lethal aid to the contras. White House officials said yesterday they were aware of continued discussions in Congress about a compromise but were planning to wait until after the president's nationally televised address Sunday night before deciding whether to respond to the suggestions.

In a new proposal, a group of swing-vote House members led by Rep. James C. Slattery (D-Kan.) asked Reagan to call a 75-day cease-fire for negotiations between the Nicaraguan government and its opposition.

The plan, similar to one endorsed by eight Latin American foreign ministers, would comply with two key Nicaraguan demands: that contra attacks stop before talks begin, and that negotiations exclude the contras. Slattery said two leaders of the contras' United Nicaraguan Opposition (UNO) group, Arturo Cruz and Alfonso Robelo, had endorsed the plan and the third and strongest, Adolfo Calero, "has not condemned it."

"Should the Sandinistas fail to respond constructively," Slattery's letter to Reagan said, "it would then be clear to all that the failure . . . should be attributed to their intransigence. At that point you would unquestionably receive greater support, at home and abroad, for tougher options."

Reagan, traveling to the State Department to view weapons captured from communists in El Salvador, warned that if the contras are not aided, "Americans will in the not too distant future look to the south and see a string of anti-American communist dictatorships." He also heard testimony from two Sandinista opponents and a former Salvadoran guerilla leader and viewed U.S. weapons captured in Vietnam and funneled through Cuba and Nicaragua to the Salvadoran rebels. During the presentation Defense Secretary Caspar W. Weinberger appeared to doze off.

Also yesterday, Rep. Tony Coelho (D-Calif.), chairman of the House Democratic campaign committee, charged that the White House was using a briefing by Reagan Friday on the contras to help GOP challengers against Democrats this fall. But White House political director Mitchell E. Daniels Jr. said Coelho was "misinformed" and the briefing was "strictly routine."

Meanwhile, at an appropriations hearing, Secretary of State George P. Shultz clashed with Sen. Arlen Specter (R-Pa.), who asked whether Reagan "is seeking a military victory for the contras." Shultz said the goal is "a rearrangement of the way of governing Nicaragua" through "a church-mediated policy of national conciliation" and contra forces who would "make it plain that the communists cannot win a military victory."

"I don't understand your answer and I don't understand the administration's policy," Specter said, adding that Shultz's reply "sounds to me as if it's a 'yes' answer." Shultz responded, "I'll make a deal with you: you don't put words in my mouth and I won't put words in your mouth."

"I know what a question is and I didn't get an answer," Specter said.

Shultz said he was "frustrated" because the question is "very difficult for me to just answer head-on, yes or no." Nicaragua is "the principal stumbling block to peace," he said, and "we want to try to turn that situation around."