The Nicaraguan rebel movement, at a crucial point in its war against the Sandinista government, has begun an intense new effort to reach the elusive goal of unity and broad political backing.

The campaign, pursued in meetings here in the past several days, has acquired particular importance as Congress and the Reagan administration cast about for ways to agree on renewed U.S. funding for the guerrillas and their antigovernment attacks in Nicaragua.

Rebel leaders have expressed the belief that a unified insurgency, rather than several competing groups, would have a better chance of support in Congress. As in the past, however, this has proved impossible. Negotiations have centered instead on a document spelling out shared political goals for the time when, the rebels hope, the Sandinista government will be overthrown.

The current effort also has focused on drawing in prominent political exiles to lend their names, if not to the armed insurgency, at least to such a political document. In this, rebel leaders have sought to imitate the Sandinistas, who enlisted unarmed political figures to back their guerrilla war that overthrew Anastasio Somoza in 1979.

The tactic has become more viable since the Nicaraguan elections in November, which in the eyes of many Nicaraguan exiles closed off the last effective internal political avenue of anti-Sandinista agitation. Adolfo Calero, who heads the largest guerrilla group, has expressed hope that the election results will persuade more exiles that his warfare is necessary.

Arturo Cruz, who for a time was an opposition presidential candidate last fall, has declared readiness to subscribe to the document spelling out political goals shared by the guerrillas and their unarmed allies. This has been sought for some time by Calero and other rebel chiefs because of what they say is Cruz's influence among congressmen generally hostile to CIA funding for the cause.

Cruz has been a key actor in meetings here among the top rebel leaders. Most have come to Miami in conjunction with a rally today organized by U.S. backers and the Council for Inter-American Security, a conservative Washington political group.

In a measure of the difficulties, however, only two top leaders, Fernando (Negro) Chamorro and Steadman Fagoth of the Indian group Misura, participated in the speeches and exhortations for congressional and private financial support.

Some rebel and congressional sources have suggested that the document also could define a political leadership one tier above the guerrilla leadership, providing a vehicle for straightforward U.S. funding rather than technically covert CIA funding for the military actions.

After appropriating about $80 million since 1981, Congress halted CIA help for the rebels last spring. In a compromise with the administration last fall, it appropriated $14 million but made it conditional on another vote of approval this spring. With indications that such approval is unlikely, administration officials have been seeking alternate ways to fund the insurgency.

But Cruz, in a telephone interview, said he plans to maintain his refusal to join the guerrilla groups, either directly or disguised through a two-tier arrangement.

"There is no intention at this point in time of any organic alliance or any umbrella" political group, he declared.

Cruz, a former government official and ambassador for the Sandinistas, consistently has avoided what he calls "organic" links to the rebel groups. He urged renewed aid for the war last month, however.

Another widely respected figure expected to join the declaration without joining the guerrilla movement is Pedro Joaquin Chamorro, who ran the opposition newspaper La Prensa until leaving Nicaragua late last year.

Their addition, along with what rebel sources said will be about two dozen other exile figures, would be the chief accomplishment of the documents. With the exception of Eden Pastora, the main guerrilla leaders already signed a joint pact on political goals last July in Panama, after what Pastora said was strenuous urging by the CIA.

Calero had expressed hope earlier that other leaders of the Democratic Coordinator, a coalition of anti-Sandinista groups inside Nicaragua, would join Chamorro in exile, strengthening his contention that armed struggle is the only effective response to the Sandinistas. But these opposition leaders have decided instead to remain in the country, he acknowledged in an interview.

Cruz and Chamorro have been urging the charismatic Pastora to add his signature to the political document as well. Pastora adamantly has refused to join forces with the main guerrilla group, Calero's Nicaraguan Democratic Force, on the ground that it contains officers from Somoza's National Guard and thus can never inspire a following among the Nicaraguan people.

Pastora's former partner in the Costa Rica-based Revolutionary Democratic Alliance, Alfonso Robelo, split off last summer and signed a unity pact with Calero. Since then, Robelo has abandoned military struggle and concentrated on political directions such as those discussed here this weekend.

Fernando Chamorro, who also led his small group into alliance with Calero, said expected military help from Nicaraguan Democratic Force coffers has not materialized, leading to some resentment. But he declared that the guerrilla alliance, backed by the political alliance under discussion, remains the best vehicle for soliciting private funds and renewing CIA funds.

As in the past, Pastora resisted efforts this weekend to link him directly or indirectly to the Nicaraguan Democratic Force, according to Chamorro and other insurgent leaders. The resistance was particularly strong because of what Chamorro said were unfulfilled pledges made during the efforts last spring to entice Pastora into the Panama pact.

Aside from aid, these concerned objections from Pastora, Chamorro and Robelo to the presence in the Nicaraguan Democratic Force leadership of Ricardo Lau, a former national guard officer considered particularly loyal to Somoza during the Sandinista civil war.

"This has been a negative element in the negotiations," Chamorro said.

Pastora also has objected to Enrique Bermudez, a former National Guard colonel who is the Nicaraguan Democratic Force military commander and one of its chief political leaders. Calero has refused to eject Bermudez, who has played a prominent role in the anti-Sandinista rebellion from its beginning.

According to Chamorro, Bermudez has been key in the movement because he forged close links with U.S. intelligence officials when he led a Nicaraguan contingent in the Organization of American States forces that invaded the Dominican Republic with the United States in 1965 and when he served as Somoza's military attache in Washington.