MIAMI (AP) _ Contra rebel officials, beset by months of squabbling, are reorganizing and broadening their leadership in an effort to attract support from more U.S. leaders and Nicaraguans.

But some U.S. congressional and rebel officials doubt the changes will make much difference.

Plans to regroup the fractious Contra umbrella group, United Nicaraguan Opposition, include nearly doubling the size of its political assembly to about 54 members.

A larger directorate would replace the three-man board crippled by the recent resignations of two original members and announcement by the third that he also will step down.

The effort to establish political unity comes as the U.S.-supported rebel forces are attempting to prove themselves militarily, with an eye toward September when Congress plans to consider the Reagan administration's request for $105 million in new aid.

As proposed, the new Contra directorate would have perhaps seven members representing different Nicaraguan political constituencies and other Contra factions, including the Miskito Indians and the Southern Opposition Bloc, known as BOS.

But the political maneuverings during recent meetings in Miami, Washington and Central America are viewed with skepticism by some Nicaraguan and congressional players, who believe the organization will remain under the domination of the military contingent led by former UNO director Adolfo Calero and military commander Enrique Bermudez.

Calero and Bermudez's Nicaraguan Democratic Force is the largest rebel army, claiming more than 16,000 members.

''The question is will the directorate be a bunch of dummies or will they really be a directorate? In order to not be a dummy, you have to have your own way of thinking, you have to be independent,'' said Arturo Cruz, an original UNO director who resigned in March complaining the body was run by a ''clique.''

He was known to have been frustrated by the Calero faction's dominance and resistance to reforms and power-sharing. That also led the third director, Alfonso Robelo, to recently state he would not remain on the new board because Calero would undoubtedly be a director.

Robelo said Calero controlled the Contra finances and would not show the books to Robelo and Cruz, who were viewed as political moderates with considerable support on Capitol Hill and in the State Department.

Brooklyn Rivera, an Indian rebel leader from Nicaragua's Miskito coast, contended: ''The FDN will control the directorate, basically, with some new faces. ... It will be a cosmetic change.'' It appears unlikely that Rivera, who is usually at odds with UNO, will join the new organization.

His skepticism was echoed by some congressional staff critics and Nicaraguans, including Enrique Gabuardi, the representative in Miami for Nicaragua's Liberal Independent Party.

''UNO is nothing. UNO doesn't represent the sentiment of the Nicaraguan people. It was an assembly created by the Americans, by the CIA, by the State Department,'' he said, holding no optimism about the possibility of change.

The State Department has been keeping a relatively low profile in the reorganization, but U.S. officials have met with some leaders and would like to see Robelo remain, said a department official speaking on condition of anonymity.

The official raised the possibility Calero might not be on the new directorate, but said: ''Our position is we are not as directly involved as we used to be. We are not engineering this.''

Robelo said, ''There has never been so little interference from the Americans as in the last two months.''

Robelo has insisted he is not a candidate, but said on Friday that Calero has been urging him to change his mind. They met over the weekend with other UNO representatives in Miami.

Cruz said he has no interest in rejoining the leadership, which is to be named by the new assembly within about two weeks.

Meanwhile, Calero, who resigned from the UNO board in February due to the increasing tensions, said he wants to rejoin the leadership and is ''ready to work with anyone.''

The new board, he said, ''will be more balanced, more representative, more widely accepted from within the Nicaraguan community, more appealing to the people inside Nicaragua.''

The UNO reforms Cruz and Robelo wanted, such as broader representation, unification of military factions into one rebel army under civilian control, and centralization of Contra finances, will be goals of the new board, said Pedro Joaquin Chamorro, who replaced Calero as a director, with Calero's endorsement.

Chamorro said he wants to remain a director, but will not be a ''rubber stamp'' for Calero.

The Contra forces claim to have more than 10,000 troops operating inside Nicaragua. They say that this year they have shot down or damaged eight Sandinista helicopters and ambushed 50 transport trucks, as well as attacked numerous ''garrisons'' and cooperatives that they contend are military posts.

The claims could not be independently verified because the Sandinistas, who put the number of Contra forces at 6,200, restrict travel to battle areas. The Contras and Sandinistas are both known to exaggerate their claims.