A fresh infusion of United States aid to Nicaraguan rebels has increased pressure on the Nicaraguan government. But it is unlikely to enable the rebels to overthrow the ruling Sandinistas or force them to change their course radically, according to informed analysts in the region. Unless the Reagan administration is willing to take the big risk of a US invasion, it probably will not be able to oust the Sandinistas, these analysts say.
``As long as the contras do not represent a real political alternative they have no future -- no hope of taking power -- unless they are backed by a US invasion,'' says one politically moderate Nicaraguan analyst.
This view contrasts with the public statements of at least one Reagan administration official working with the rebels who says the increases in rebel strength will enable them to overthrow the Sandinistas on their own.
The rebels themselves, called ``contras,'' have high hopes that they will now be able to deal the Sandinistas heavy blows, contra sources say.
Last week, President Reagan signed an aid package providing an additional $27 million in nonmilitary aid to the contras.
When Congress approved this aid Aug. 1, it reversed its rejection three months ago of renewed assistance. Administration officials have told the press they are confident that Congress's move would be the first in a series of such approvals. These officials and contra sources say that with it the contras should be able to increase their force to some 25,000 fighters.
At present, contra sources claim they have 16,000 to 17,000 men in Honduras, in addition to 2,400 Miskito Indians and several hundred other fighters active in Costa Rica. Western diplomats in Managua and other independent sources put contra numbers at closer to 12,000 to 13,000, including the several hundred men fighting under rebel leader Ed'en Pastora in Costa Rica. They say that Miskito forces may bring total contra strength to around 14,000.
These diplomats doubt that the contras will be able to put 25,000 men into the field within the next year or so. The contra forces will grow by 5,000 or 6,000 at the most, the predict.
But, however large, contra forces will face a Nicaraguan Army estimated at 64,000, plus more than 50,000 militiamen.
The pressure being brought on the Sandinistas with the fresh aid to the contras is largely economic. One Nicaraguan economist who is neither pro-San- dinista nor pro-contra, emphasized that while the Soviets were giving the Nicaraguans arms and indirect financial aid in the form of oil, they were probably not giving them large financial subsidies to maintain the Army, which gets 40 to 50 percent of the Nicaraguan budget.
``In the end,'' the economist said, ``supplies for the Army have to be bought abroad in dollars -- which the Sandinistas have to pay for in cordobas [the Nicaraguan currency]. With the cordobas now at 800 to $1, this will eat away more and more Sandinista financial resources.'' There were roughly 10 cordobas to the dollar at the time of the 1979 revolution.
US administration officials and contra leaders hope this increase in the budget for the Sandinista Army will translate into greater economic hardship for the Nicaraguan people and, thus, greater discontent and anti-Sandinista feeling among the population.
Western analysts in Nicaragua do see a steady slippage of popular support for the Sandinistas. But they do not see ``a massive crumbling,'' as one Western diplomat put it, nor do they believe that such a breakdown of support will take place within the next year or so.
The political discontent with the Sandinistas does not translate into support for the contras, says the moderate Nicaraguan analyst. The contras, he says, have not convinced the Nicaraguan people that they are a viable alternative -- they are seen as ``pro-Somocista, pro-Yankee, and pro-rich people.''
Western diplomats believe that although the contras will be able to spread more fear by being able to operate in more central parts of the country, they will not be able to hold territory in the foreseeable future.
The most important factor in the Sandinistas' favor, according to Western analysts in Managua, is the increasing strength and efficiency of the Sandinista Army, which has up to now been able to successfully deal with contra attacks.
Sandinista strength has been greatly increased by the delivery of Soviet MI-8 and MI-24 armored helicopters, which will give the Army much more mobility in combating the contras.
``These things are like flying tanks, but tanks that will be able to get the Sandinista military on the spot quickly whenever there is a contra attack,'' a Western analyst in Managua said.
Western diplomats in the area do not believe that the five or six small DC-3 planes the contras are acquiring will be able to make up for the impact of the new Soviet helicopters.