The White House searched anxiously yesterday for a way to give Congress a greater say in determining whether more lethal aid will be sent to the contras in Nicaragua amid growing signs from Capitol Hill that the request will be defeated Wednesday.

House Majority Leader Thomas S. Foley (D-Wash.), usually the most cautious member of the leadership, said flatly that the opponents would defeat President Reagan's request for more aid. "We're going to win," Foley said.

President Reagan is to outline that request tonight in a television speech, but as of yesterday afternoon only Cable News Network had agreed to carry the address live. The three major networks were not expected to decide until today whether to carry the address, and Democrats were reported to be demanding equal time to answer the president.

White House strategists consulted yesterday with congressional leaders over prospects for the request as the president went before a convention of religious broadcasters here to plead that the aid is essential to "keep the pressure" on the Sandinista government.

The broadcasters cheered the president's remarks and laughed when he slipped in his speech and told them that the Sandinista government had made some "hopeful confessions, concessions I should say" toward peace in Central America.

But as the group applauded the president, House Democrats were expressing confidence that they will be able to defeat Reagan's contra aid request. They also said that even an 11th-hour pledge by the president to give Congress a greater role in triggering the release of lethal aid funds would not change the outcome.

"I don't think it would be credible," said Democratic Whip Tony Coelho (D-Calif.). "They're playing with the trigger, hoping that the trigger will be the issue. But the reason they're in trouble is not the trigger. The reason they're in trouble is the military aid."

"There are a number of ways to approach this," said White House spokesman Marlin Fitzwater, sounding conciliatory toward Congress. "But suffice to say, we do want to involve the Congress as much as possible."

Rep. David E. Bonior (D-Mich.), who heads his party's task force on the contra aid issue, said that congressional involvement in triggering the release of funds for lethal aid is less an issue with rank-and-file lawmakers than the administration's definition of "nonlethal" assistance and the growing feeling in Congress that the administration's request is incompatible with the Central American peace process.

"The problem is with the administration's definition of nonlethal aid," said Bonior. "They have bastardized the language so much."

The $36.2 million package the House will vote on Wednesday includes $3.6 million specifically earmarked for lethal aid, and the remainder for nonlethal assistance, which the administration defines as anything but "weapons, weapons systems and ammunition." Under the president's request, the $3.6 million in lethal aid would be placed in escrow during March and then released if Reagan certifies that a ceasefire is not in effect.

It also provides for placing an additional $20 million into a fund for replacement of any leased aircraft that are shot down, and for the transfer of an unspecified amount for "passive air defense equipment" such as radar to ensure the safety of aid flights.

The House Democratic Study Group estimates that the electronic gear could be worth between $4 million and $12 million, bringing the total cost of the package to between $60 million and $68 million.

The House Democratic leadership has been pushing the argument that the administration's request includes little true "humanitarian" aid of the sort that is favored by many lawmakers who do not object to providing additional food, clothing, medicine and shelter to the Nicaraguan rebels. Only $7.2 million of the administration package is for such aid, according to one congressional aide.

The administration's definition of "nonlethal" assistance, said Coelho and Bonior, includes materiel to press the war effort such as jeeps, trucks, the leasing of helicopters and the transportation of troops. The level of that assistance, they said, represents an escalation because it nearly triples the current level of nonlethal assistance.

"No matter how you camouflage it, it is still an aggressive effort," said Coelho, who accused Reagan of "escalating at the very time all parties are saying . . . there is positive progress" toward peace.

House Democrats have bolstered their case by promising key moderate "swing" voters that once the administration package is defeated they will seek a humanitarian aid package of their own that might also include broad-based economic assistance to the entire Central American region.

House Speaker Jim Wright (D-Tex.) said yesterday the details of that alternative would not be developed until after Wednesday's vote.