Congressional opposition is rising to the Reagan administration's not-so-secret war in Nicaragua, one of the most controversial U.S. undercover operations of recent years.

CIA support of paramilitary operations in and around Nicaragua goes back to an authorization by President Reagan in March, 1981, according to authoritative sources. But the secret activity took a new turn with an expanded presidential order in November, 1981, that brought about a sharp increase in U.S.-backed military activity in Nicaragua and, here at home, a sharp increase in press scrutiny, public discussion and congressional uneasiness.

Since its cover was blown in Washington press reports early last year, the "secret war" has been the subject of countless newspaper articles, a magazine cover story, television evening news reports, heated debate in the Senate and House and, since Dec. 21, a highly unusual provision of U.S. law limiting the purposes of the operation.

The legal restriction is known as the Boland amendment, after House Intelligence Committee Chairman Edward P. Boland (D-Mass.), its sponsor in the House. This law says CIA or Defense Department money cannot be spent to support irregular military activities "for the purpose of overthrowing the government of Nicaragua or provoking a military exchange between Nicaragua and Honduras."

A growing number of members of Congress, including senior members of the committees overseeing intelligence matters, are concerned that the spirit and possibly even the letter of the law are being violated. Hearings in secret and perhaps in public are likely soon after Congress returns from Easter recess, and chances appear strong that U.S. support of the undercover war will be restricted further or even banned.

Daniel Patrick Moynihan (D-N.Y.), vice chairman of the Senate Intelligence Committee, said Friday that "a crisis of confidence" is building over this issue between Congress and the intelligence agencies. "It is absolutely necessary that the administration obey the law," said Moynihan, who expressed the view that either the law or the operations must be changed because the current situation is untenable.

The statute, which gave legal standing to an earlier secret directive of the Senate and House Intelligence committees, was adopted in the House in a unanimous 411-to-0 roll call vote. Yet, according to Moynihan, there is "evidence every night on television" that the law is being violated.

Another influential Senate Intelligence Committee member, who asked not to be quoted by name, said, "The quiet cloakroom debate going on is: do we really want to enforce the law?"

Across Capitol Hill, 37 House members headed by Rep. George Miller (D-Calif.) wrote President Reagan to express their concern that the Boland amendment is being violated and to urge "strict compliance" with the law.

One of the signers of the letter, Chairman Michael D. Barnes (D-Md.) of the House Foreign Affairs subcommittee on Latin America, said CIA attorneys argue that continued spending is legal because the "purpose" of the U.S. agencies supplying money and weapons to the insurgents is not to overthrow the Nicaraguan regime, even if the purpose of the guerrillas who receive the support is to do so. "Not a jury in the country would accept this, and the House will not accept it," said Barnes.

The Carter administration reportedly began secret U.S. aid to democratic, non-communist forces in Nicaragua prior to the fall of Anastasio Somoza and the victory of the Sandinista revolution in July, 1979. Those centrist forces did not prosper as Nicaragua turned increasingly to the left.

The Reagan administration came to power with a more hostile view of the Nicaraguan regime, based in part on Reagan's ideology and in part on the belief that the Sandinistas, with Cuban backing, were exporting arms and revolution to their Central American neighbors. On March 9, 1981, according to informed sources, Reagan approved an official "finding," as required by reformist laws on intelligence activities, that a secret program of arms interdiction by the CIA in Central America was "important to the national interest."

About $19.5 million a year was allocated to this program, which is believed to still continue. Compared with what came later, these activities were relatively uncontroversial among Congress' Intelligence committees and others who know of them, since they were seen largely as a secret U.S. response to secret communist military activity.

In the course of 1981, the administration was increasingly frustrated by the military stalemate in El Salvador, Guatemala and other contested areas of Central America. By late fall, Reagan had shelved as too dangerous a plan by then-Secretary of State Alexander M. Haig Jr. involving U.S. naval and air action against Nicaragua and Cuba.

About the same time, the administration closed the book on confidential August-to-October diplomatic exchanges with Nicaragua by Assistant Secretary of State Thomas O. Enders, who had been seeking Sandinista agreement to stop support of revolutionary activity in the region in return for improved relations with Washington, including U.S. economic aid.

In light of those developments, a National Security Council meeting on Nov. 16, 1981, forged a new plan for Central America, including stepped-up U.S. economic and military aid to several countries, tightened economic sanctions against Cuba and "contingency planning" for the use of U.S. military force in case of "unacceptable military actions" by Cuba.

As part of this 10-point program, Reagan authorized for the first time CIA support for "political and paramilitary operations" in Nicaragua, not limited to the interdiction of arms but with the broad purpose of combating the Sandinista regime. A paramilitary force of about 500 men was initially envisioned, to be supplemented by about 1,000 men already reportedly being trained by Argentine officers.

This program, also costing $19 million, was formally approved by Reagan as National Security Decision Directive 17. A few days later, at the beginning of December, Reagan signed a new presidential "finding" to justify the new secret activities in Central America, in turn triggering confidential reports to the Intelligence oversight committees of Congress.

The CIA, in presenting its plan, specifically told the president that the "opposition front" to be built in Nicaragua would be anti-Somoza as well as anti-Cuban. Although remnants of the late dictator's National Guard were still intact in the area, these forces were considered too unpopular to have much credibility or chance for success.

In the field, however, U.S. operatives found that the only organized and trained groups capable of serious armed opposition were those that had backed Somoza and had been driven out by the Sandinistas. To create other militarily viable groups would be to start down a long and uncertain road.

By default, therefore, Washington was driven into backing former Somocista military elements, although the most notorious and ineffective figures in these groups were purged following the U.S. connection. This reliance on ex-National Guard forces has created problems for the "secret war" among potential allies in the field, including former Sandinista hero Eden Pastora, who has re- fused to join up, and controversy among some in the U.S. intelligence agencies.

In February and March, 1982, as military action began to heat up in the region, press reports in Washington revealed Reagan's secret sponsorship of the drive.

From the start there was ambiguity about the purpose of the U.S.-supported paramilitary effort: Was it to overthrow the Sandinista regime through civil conflict or in a war with Honduras, where most of the insurgents were based? Was it to increase the costs and risks to Nicaragua for supporting revolution outside its borders, with the implication that this U.S. pressure could be relaxed if Nicaragua halted its aid to rebels in El Salvador and elsewhere?

Was the action to interdict the flow of arms and other supplies, even if Nicaragua continued to dispatch them? (This is the purpose of the separate CIA program, but the two have been repeatedly and possibly deliberately confused.)

According to Moynihan, the administration justified the undercover activity by say- ing it was "designed to produce a more liberal government in Managua." Moynihan does not find this idea credible.

His concern, and that of other members, generated a closed-door debate in the Intelligence committees last summer. In August, Senate and House conferees attached a statement to the intelligence authorization bill limiting the purpose of the "secret war" in the same words later adopted as the Boland amendment.

Few people were aware of the committees' concern or of their directive at the time, because the discussion was secret and the bill and the accompanying conference report are also secret. While these documents are open to inspection by any member of Congress, very few lawmakers outside the Intelligence committees take the trouble to look at them.

Rep. Tom Harkin (D-Iowa), who led a legislative drive last December to halt U.S. support for the undercover war, said he learned of the Intelligence committees' prior action only on the day of the House floor debate on the issue.

At that point Boland publicly revealed what had been done earlier, successfully appealed for its adoption as law by the House, and assured his colleagues that the Intelligence Committee "certainly does understand its obligations to rein in activities which can get out of control or which could threaten to involve this nation or its allies in a war."

Since the December legislative action, reports from the field indicate that the insurgency in Nicaragua has become stronger and that its attacks are bolder and more numerous. It is considered increasingly a credible threat to the Sandinista regime.

In the face of attacks a year ago, the government in Managua imposed a state of emergency with harsh political and security controls. If its survival is more seriously threatened, according to some Washington assessments, the Sandinistas might well call for more concerted Cuban and Soviet back- ing, possibly including Cuban military units. This, in turn, could bring a U.S. response, leading to a major military confrontation in Central America.

Controlling and calibrating the undercover war in U.S. national interests, as determined in the administration, is anything but simple for Washington officialdom. The arrival of Congress on the scene in major fashion may add a complicating and, perhaps, controlling element.