Covert U.S. support for guerrillas fighting the leftist Sandinista government in Nicaragua has strengthened international support for the Sandinistas and has failed to stop them from helping to train and arm leftist rebels in nearby El Salvador, the Democratic majority of the House Intelligence Committee said in a report released yesterday.

The Reagan administration has "allowed the spotlight of international opprobrium to shift from Sandinista attempts to subvert a neighboring government in El Salvador to a U.S. attempt to subvert that of Nicaragua," the committee's nine Democrats concluded in the unprecedented report about an ongoing CIA covert operation.

Significantly, they agreed with the Reagan administration's contention that the Sandinistas have helped give communist-backed rebels in El Salvador logistical support, training and arms.

They stated in the report that intelligence information shows "with certainty" that "a major portion of the arms and other material sent by Cuba and other communist countries to the Salvadoran insurgents transits Nicaragua with the permission and assistance of the Sandinistas."

This arms flow has not been interrupted, the Democrats concluded, by covert CIA support for about 7,000 anti-Sandinista guerrillas in Nicaragua. "The acid test is that the Salvadoran insurgents continue to be well armed and supplied," the report said. "They have grown in numbers and have launched more and longer offensives. All this requires an uninterrupted flow of arms."

"There are certainly a number of ways to interdict arms, but developing a sizable military force and deploying it in Nicaragua is one which strains credibility as an operation only to interdict arms," the Democrats said in the report.

However, the committee's five Republicans issued a dissent stating their conclusion that the covert operation has been successful in deterring arms shipments.

Cutting it off, as the committee's Democratic majority has voted to do, would hand "a legislatively engineered victory" to the Sandinistas, the Republicans said.

"The Sandinista Nicaraguan government marks the first foothold of Marxism on the mainland in our Western Hemisphere," they said. "With only a modicum of help from the United States democracy can flourish in Central America."

The Democrats argued in the 44-page report that U.S. support for the anti-Sandinista insurgents has had "entirely opposite results" from those intended. "Having twice sent U.S. troops to Nicaragua in this century, this country has once again been cast in the role of interventionist."

Citing congressional testimony by Assistant Secretary of State Thomas O. Enders, the House committee concluded that Cuban agents brought rival Salvadoran guerrilla factions together, worked out a unity pact and set up Salvadoran rebel headquarters in Managua.

Nicaragua and Cuba appear to be continuing their training of Salvadoran rebels, the committee said.

According to its report, in December 1981, five days after the committee's first briefing on the program by administration officials, Chairman Edward P. Boland (D-Mass.) wrote "the principal executive branch briefer"--CIA Director William J. Casey--raising questions about the number and tactics of the anti-Sandinista Nicaraguan guerrillas, the extent of U.S. control, and the possibility of military clashes between Nicaragua and Honduras, where the rebels had established bases.

A few months later, the committee adopted classified language in the intelligence authorization bill limiting the covert operation to interdicting arms to the Salvadoran rebels, rather than to overthrowing the Sandinista government in Nicaragua. In December, the same language, known as the Boland amendment, was publicly attached to a defense appropriations bill on the House floor.

Last week, the committee voted to terminate the covert operation and substitute assistance to "friendly countries" like El Salvador and Honduras to try to stop the arms flow to Salvadoran rebels. The report released yesterday accompanied this legislation to the House Foreign Affairs Committee, which is to consider it Wednesday.