President Reagan's renewed efforts to obtain arms and financial aid for the "freedom fighters" opposing the leftist government of Nicaragua is based on a grim assessment that the rebel cause faces defeat both on the battlefield and in Congress, administration officials say.

"There's a feeling here in the White House that we've got a lot of work to do to build support for the freedom fighters," said one official.

Reagan, who rarely takes initiatives within his administration, was described by aides as taking a lead role in an uphill effort to persuade Congress to provide $14 million in aid for the Nicaraguan rebels, called contras.

The president was described as being so emotionally committed to the anti-Sandinista campaign that he wanted to devote his radio speech to the issue on Feb. 2, the Saturday before he sent his budget to Congress. His dismayed advisers talked him into speaking on the budget instead.

Reagan insisted, however, that Nicaragua be given high priority and that the issue of Soviet involvement be addressed directly despite the administration's attempts to improve relations with the Soviets in other spheres.

On Feb. 16, in a radio speech from his California ranch, the president called the rebels "our brothers" and said "we cannot turn from them in their moment of need." In a phrase contributed by his national security affairs adviser, Robert C. (Bud) McFarlane, he linked the struggle of the Nicaraguan rebels to "freedom fighters" opposing the Soviets in Afghanistan and other leftist regimes around the globe.

Last Thursday, at his news conference, Reagan repeated this idea and came closer than he ever has to calling for an outright overthrow of the Managua regime. The president said he favors removal of the regime unless it admits the rebels into the government.

The campaign on behalf of the contras continued Friday when Secretary of State George P. Shultz, in a speech to the Commonwealth Club in San Francisco, said that "the democratic forces in Nicaragua are in the front line in the struggle for progress, security and freedom in Central America."

These speeches were the opening salvo in a campaign that White House officials say will continue through spring, with perhaps time out in early March when the administration focuses on another favorite proposal of the president's, the MX intercontinental ballistic missile.

"There is no fleshed-out strategy for exactly what we do," said one knowledgeable White House official. "But before the summer is over, maybe before the spring is over, members of Congress will have an opportunity to stand up and be counted on the future of Central America. That's the way the president's going to take the issue to the public: We're for democracy, support the friends of democracy. But we have a steep hill to climb."

This official said that the administration still believes that "covert aid" is the most effective response in Nicaragua but that the president is saying to Congress: "If you have a better way to help, let us know."

Some officials expressed an almost desperate sense that time is running out on administration policy in Nicaragua. They said the president, usually optimistic, was concerned that the contras would be defeated in the field and that the future of democracy in Central America would die with them unless the United States provides more financial aid within the next few months.

One aide says the president has no illusions that Congress will be persuaded to change its policy, but added: "He thinks that it's important enough to try."

Officials gave several reasons, which they said converged, for the timing and intensity of the present campaign.

The first was intelligence reports indicating that the Sandinistas are going to make a major effort to wipe out the rebels, who are reportedly short of ammunition, within the next few weeks. Administration officials say this is why the Sandinistas are massing troops and tanks at the Honduran border; the Nicaraguan government says it is because they fear U.S. invasion.

Regardless of the course of the war, the administration expects that the Sandinistas will begin peace overtures when aid to the rebels comes up for a vote in Congress.

Another spur to the administration was McFarlane's report to the president, after a trip to Central America late last month.

"Bud found that other countries were concerned about the Nicaraguan military buildup and concerned that the United States might leave them in the lurch," said one official. "Reagan and Shultz are sending a message to our friends in the region, which is that we can't send money because Congress won't let us but the president is willing to put his prestige and reputation on the line to help."

This report coincided with what officials said has been an increasingly successful public relations campaign on behalf of the rebels led by Otto Reich, the Cuban-born State Department coordinator for public diplomacy in Latin America.

Reich was chosen by William P. Clark, McFarlane's predecessor as Reagan's national security affairs adviser. He is viewed at the White House as having provided an effective middle ground between the militant and controversial "Central America outreach program" waged the past year by White House public liaison Faith Ryan Whittlesey and State Department officials who were regarded as insufficiently concerned with the Nicaraguan rebels.

Whittlesey, probably the most outspoken administration official on behalf of aiding the contras, said Friday in Costa Rica, where she was attending a foreign policy conference, that "there is overwhelming support here for the president's position in aiding the freedom fighters and great fear about what is happening in Nicaragua."

Officials were unanimous in saying that Reagan's own commitment has been the decisive element in the renewed Central America campaign.

"He means it when he says he isn't going to lose a country to communism on his watch, and he would like to see Nicaragua become a democracy again," said an official.