Contras' greatest challenge: boosting credibility. The Nicaraguan contras are in a race against time to prove their worth. They face an uphill battle on two fronts - on the ground in Central America and in the corridors of the US Congress.

The contras are under siege. They are anxiously awaiting new revelations about the Iran-contra scandal. They are battling desperately to prove themselves a serious force. They are seeking to counter persistent doubts about their politics.

But central to all the problems of the United States-backed Nicaraguan rebels, in Nicaragua as in Washington, in Miami as in Central America, is a critical stumbling block: a lack of credibility.

``We are simply seen as mercenaries,'' says Adolfo Calero, a contra leader. ``We have to project ourselves as winners if we are to have more success.''

In the six years since the Central Intelligence Agency first nurtured the seeds of a Nicaraguan counterrevolution, the US has spent hundreds of millions of dollars on the contras. But to dozens of current and former rebel officials, diplomats from the region and beyond, and US congressional sources, the contras do not look like winners, now or in the foreseeable future.

Not for lack of fighters, however. The United Nicaraguan Opposition, the contras' political front, claims to have as many as 20,000 fighters, mostly peasant farmers. Though this is clearly an exaggeration, Western military experts say, even the more reliable estimate of 10,000 to 12,000 soldiers gives the contras a far wider base than other guerrilla groups in the region.

But the mountains of Nicaragua, where Army patrols stalk rebel infiltrators, are only one front in the contra war. The other battlefield, where United Nicaraguan Opposition (UNO) leaders have concentrated their main efforts, is in Washington, where rebel officials stalk White House aides, congressmen, and journalists.

For the contras, Washington means sustenance. Without it, their future would be ``dim and grim,'' UNO spokesman Ernesto Pal'acios says. Although the ultimate aim is to overthrow Nicaragua's Sandinista government, the battle to ensure continued US support is the contras' top priority, he says.

``We have to win the political war in the United States, and then the military war comes later,'' Mr. Pal'acios says. ``What's the point in winning a battle, and losing the support of the US Congress?''

Since the Central Intelligence Agency began organizing a handful of former dictator Anastasio Somoza Debayle's national guardsmen in 1981, the Reagan administration has found a variety of ways to fund the contras' growth. It has provided covert military aid through the CIA and overt humanitarian aid through Congress. It has encouraged private individuals and foreign potentates, such as the Sultan of Brunei, to contribute. And, apparently illegally, former National Security Council aide Oliver North diverted profits to the contras from US arms sales to Iran, US officials say.

Dependence on US funding has been the kiss of death for the contras, critics of US policy in Nicaragua say. ``If there is going to be an opposition to Sandinismo, it has to be domestically inspired,'' a Democratic Senate aide says. ``We have poisoned the opposition.''

US aid ``has made us look artificial,'' a top UNO official agrees, citing ``an impression [that] without that money the Nicaraguan resistance would disappear.''

The impression is shared by figures within the ``resistance,'' as the contras prefer to call themselves. ``UNO can't give legitimacy to our struggle because it is an artificial creation of Washington, a US puppet,'' laments a rebel aide whose organization has refused to join UNO.

Under strong US encouragement, UNO was created in June 1985 in a bid to gather all the armed anti-Sandinista groups behind a single leadership that would be politically acceptable to the US Congress. The move failed to convince rebel leader Ed'en Pastora G'omez, whose men made up the southern front, or Misurasata, the largest Miskito Indian rebel group, which is demanding autonomy for their indigenous people on the Atlantic Coast.

The inclusion in UNO, however, of former Sandinista officials Arturo Cruz and Alfonso Robelo diluted the contras' image as a collection of ex-Somocistas. But Mr. Cruz and Mr. Robelo brought little to UNO except their credentials as well-known democratic leaders, say their supporters in Congress and in UNO itself. The real power, Cruz and Robelo soon found out, remained in the hands of two men: Adolfo Calero, leader of the largest contra organization, the Nicaraguan Democratic Force (FDN), and the FDN military commander, Enrique Berm'udez, a colonel in Somoza's former National Guard.

Tensions between FDN officials and UNO politicians broke into open squabbling last year and still persist below the surface, rebel aides say. ``The FDN sees Arturo Cruz's faction as imposed by Washington, and Cruz's people regard the others as Somocistas,'' one aide says.

But with the FDN firmly in charge of the Honduran-based contras, the Cruz-Robelo wing's influence is limited, political observers say.

``UNO is a myth,'' a congressional aide says. Adds another: ``The guys running the shop are the ones who sat at Somoza's knee. That may not be Cruz or Robelo's game, but I don't think they understand the game that's being played.''

``We Nicaraguans are terribly divisionist,'' admits a top UNO leader. ``We lack the discipline to acknowledge that `you're not a saint, I'm not a saint, but we have a common enemy.'''

But that common enemy is sometimes lost from view. ``We are struggling in two directions, against the Sandinistas, and among ourselves,'' a rebel official says. ``And sometimes the struggle is more passionate among ourselves.''

The struggle has focused on issues such as civilian control over the contras' military forces and on the importance of developing a coherent political strategy, which has been slow in coming.

``The contra operation was run backwards,'' says a political analyst with ties to the rebels. ``They put a military structure together, then politics, then an international effort.''

One UNO official agrees. ``The emphasis on the military aspect [of the fight against the Sandinistas] has been a mistake,'' he says. Other rebel aides, however, say that the contras deliberately avoided setting out their political goals for fear of sparking internal friction.

``The FDN has never wanted to do political work,'' says Carlos Icaza, the FDN official in charge of legal affairs. ``We have people with different political attitudes in the leadership and at the executive level, and to establish one political line would be to lose that variety.''

Pressure from the more politically minded contras, though, has led to the publication of a general set of political principles, laying out a basically conservative government plan. Released this month, the plan envisions ``an authentically democratic regime,'' which stresses political pluralism, the family, and private property as ``an expression of natural law.'' Contra sources say the new document marks the beginning of efforts to broaden the rebels' appeal to Nicaraguans at home and abroad who have so far proved at best indifferent and at worst openly hostile to the contra cause.

Until recently, the contras had effectively ignored the estimated 450,000 Nicaraguan exiles in Central America and the US as a potential political base, current and former rebel leaders say.

``Not even 1 percent of the exiles are actively involved'' with the contras, says Francisco Cardenal, an FDN founder, who, because of differences with his colleagues, has since left the organization.

Washington's preeminence as the contras' main battleground means not only that Nicaraguan exiles have merited little attention but also has meant a failure to target political work at Nicaraguans living under Sandinista rule.

``With the need at the moment to win US opinion and Congress, the Nicaraguan people are secondary,'' a contra adviser says. ``No one is thinking about the masses right now.''

One result of this, says an UNO official, is that ``many people in Nicaragua think that the contras are coming to kill them,'' and the rebels are perceived throughout Nicaragua as an army of former national guardsmen.

That perception, US and contra spokesmen say, is false, because the vast bulk of rebel foot soldiers had no military training before they became contras.

But even by the US State Department's reckoning, the FDN is controlled by former guardsmen, who occupy five of the seven posts on the organization's strategic command, including commander in chief. The FDN's top four pilots, naval chief, and head of counterintelligence are also former guardsmen.

To dispel the contras' image as a force fighting to restore the ancien r'egime, UNO last week began transmitting radio programs from its new station, Radio Liberaci'on, reportedly based in El Salvador, but heard in most of Nicaragua.

Behind all the contras' visions of Nicaragua's future lie serious doubts about how they might be achieved. These doubts spring from the very nature of the rebel movement and army.

``The suffocating reality is that these people have the expectation that the Americans are going to solve their problems for them,'' says a Republican Senate aide. The rebel army, a source close to the top contras laments, ``has been trained to act as an army of occupation, after a US invasion, not as an insurgent force.''

Today, with a new emphasis on politics and renewed military training by US insurgency experts, there are signs that the contras' tactics may be changing. But with the Iran-contra affair clouding the prospects for further US congressional support this year, the rebels are in a race against time to prove their worth.

One key factor, diplomats, military experts, and contra leaders agree, will be the rebels' ability to make their mark quickly and dramatically on the battlefield against the Sandinista Army, which has an estimated 60,000 troops.

Tomorrow: The military outlook.