The Reagan administration, discarding indirect approaches, has decided to make a frontal assault on Congress over one of the few issues President Reagan appears to be losing on Capitol Hill: new funds for anti-government rebels in Nicaragua.

Several recent reports, speeches and trial balloons accompanied the decision-making process.

Ideas considered -- and apparently rejected, at least for now -- reportedly included using U.S. allies in Latin America or Asia as third-party conduits for rebel aid, as well as the possible suspension of U.S. relations with Nicaragua and recognition of a rebel government outside the country.

Showdown votes on the issue appear likely next month.

On all other top-priority issues, Congress has given or seems likely to give the president much of what he wants.

But Congress, led by the Democratic-controlled House, has four times refused further aid to rebels against the leftist Sandinista government of Nicaragua, arguing that the three-year-old U.S. aid program is illegal and the rebels' campaign brutal and counterproductive.

Faced with a margin of 50 to 60 negative votes in the House and hostile leadership in the crucial Senate Select Committee on Intelligence, where the funds must originate, the White House at first admitted that defeat was likely. Officials cast about for alternatives for two months or so, but found nothing workable.

In a series of interviews last week, however, senior administration officials said the stakes of U.S. credibility and national interest have grown high enough on this issue to constitute what one called "a line in the dust" that Reagan can draw to find out who stands with him and who does not.

As a result, they said, the president will spearhead a major "public education" effort to pressure Congress to provide $14 million for the Central Intelligence Agency to keep the rebels supplied.

"Now it's just a question of timing," one State Department official said.

Much is at stake. Terms of the debate were set in last year's budget resolution, which banned aid to the rebels unless the president reports it is necessary and wins the approval of both chambers of Congress. Once the report is submitted, votes follow automatically after 15 days, with no amendments allowed.

To lose a head-on confrontation after picking its date would be a serious embarrassment and the president's first major congressional defeat in foreign policy.

Rebel spokesmen say the need is urgent. The final CIA checks went out last May, the last of about $80 million that built the rebels from a ragtag band of about 200 in 1981 to an estimated 14,000 trained fighters.

Enrique Bermudez, military commander of the largest rebel group, the Nicaraguan Democratic Force (FDN by its Spanish initials), said last week that "the psychological and political effect" of renewed congressional support would be critical to the rebels' private fund-raising effort. "Then other friends of our cause will support us also," he said.

As far as has been determined, the insurgents have been operating on private donations as well as food, clothing and medicines that free other money for arms purchases.

But that flow will falter if Congress balks, because the rest of the world will see it "as a signal that the United States is withdrawing from Nicaragua," Bermudez said.

In addition, Sandinista forces are reported to be preparing for a major campaign to halt rebel border incursions. If the rebels are routed, administration officials have said, neighboring Costa Rica and Honduras would be flooded by as many as 150,000 fleeing rebels, family members and supporters.

Reagan has given emotional endorsements to the rebels at least nine times over the past two weeks, calling them "our brothers" and the "moral equivalent of the Founding Fathers," and urging Congress to support their drive for a democratic Nicaragua.

There has been little visible response on Capitol Hill, where critics complained that the administration has not tried seriously to negotiate a settlement with Nicaragua, either bilaterally or through multinational peacemaking efforts such as the Contadora talks launched by Mexico, Panama, Venezuela and Colombia.

Secretary of State George P. Shultz responded in testimony last week that only military pressure by the rebels can force real concessions from Nicaragua. Officials acknowledged privately that they have no other sticks or carrots to use in the discussions, which are expected to resume later this month.

A White House legislative strategy session Friday formalized the decision to abandon, at least for now, the creative financing ideas that surfaced recently as possible ways around congressional objections to not-so-secret CIA involvement with the rebels.

Curtin Winsor, U.S. ambassador to Costa Rica, had pushed informally for a suspension or a break in U.S. relations with Nicaragua, arguing that this would allow controls on U.S. merchants who now provide Nicaragua with about 60 percent of its foreign trade, according to conservative sources outside the government.

It would also pave the way for recognition of an alternative rebel government, perhaps in Costa Rica, that then could receive U.S. aid openly. The Sandinistas used this tactic to gain international backing in early 1979 for their drive against dictator Anastasio Somoza.

But the Sandinistas were much stronger and more united then than the anti-Sandinista rebels are now, even though the rebels have more troops. The idea foundered on Shultz's misgivings about the precedent it would set and the inability of the rebel factions to get together.

Rebel unity has become a major sticking point in Congress. Efforts to achieve it at a meeting in San Jose, Costa Rica, last month produced a joint "declaration of principles" but no agreement on tactics or leaders.

FDN spokesman Bosco Matamoros said that even a statement of congressional support for the declaration would be useful. "We're not even asking for the money. It's a political statement and we want a political response," he said.

Robert C. McFarlane, Reagan's national security affairs adviser, is reported to have suggested that funds could be moved to the rebels through U.S. allies in Asia, disguised as additional foreign aid. Other officials proposed similar phony aid grants to Honduras or Costa Rica under unwritten "gentlemen's agreements" that the funds would be passed to the rebels.

But members of Congress reacted strongly against the idea. "Once we vote against something, they're not supposed to go around us and continue the policy," a senior House Appropriations Committee official said.

"The lawyers made it clear to us that none of that would fly," a senior State Department official said. He said no formal proposals were made along these lines after trial balloons surfaced in the news media. "None of it ever got past chitchat over lunch," he said.

Very much aware of the decision-making effort, administration critics timed their initiatives accordingly. Nicaragua offered March 1 to send home 100 Cuban advisers and to postpone "for the indefinite future" any effort to obtain advanced MiG jets or other fighter aircraft.

Americas Watch, a private human-rights monitoring group, last week issued a report charging the FDN with attacks, kidnapings and murders of civilians, and accused the U.S. government of "major responsibility" for the alleged rebel atrocities.

A similar private report was endorsed by the Washington Office on Latin America, a Washington-based educational group that opposes Reagan's Central American policies, and by the International Human Rights Law Group, a litigating organization of attorneys and human-rights advocates in Washington. The administration immediately denounced the latter study as "bought and paid for by the Sandinistas." Matamoros denounced the reports as false and said the FDN would ask Nicaragua's permanent human-rights commission to investigate the charges.

A high State Department official said all these events have been only preliminaries to the real White House campaign, which will begin in April after Congress votes on the fate of the MX missile. "You ain't seen nothin' yet," he said.