The Reagan administration has drafted a new legal justification for covert U.S. aid to anti-government guerrillas in Nicaragua that stresses disrupting and pressuring the leftist Nicaraguan regime rather than just trying to prevent it from sending arms to leftist rebels in El Salvador.

The new administration "finding" on the need for the covert CIA operation in Nicaragua is expected to be transmitted soon to the Senate Intelligence Committee. The report goes beyond the original explanation given to Capitol Hill, according to administration and congressional sources.

Instead of seeking to stop what the administration contends is a substantial arms flow from Nicaragua to the rebels in El Salvador, the sources said, the report explains that the guerrillas backed by the United States should cause the same kind of trouble for the Sandinista regime in Nicaragua as the leftist insurgents are causing El Salvador.

"They haven't confiscated any rifles or anything like that," a senior Pentagon official said of the anti-Sandinista guerrillas yesterday. "But they put pressure on Nicaragua and divert its attention from El Salvador."

The failure of these so-called "contras" to capture arms is one factor underlying the new justification for the covert aid program, officials said. In addition, in opposing a pending House proposal that would replace the covert aid with open efforts limited to stopping the arms flow, they said the administration must explain that such an arms interdiction program alone could not work.

The new justification and the planned request for increased covert aid to the anti-Sandinista guerrillas do not reflect a new strategy, the officials said, but a clearer statement of administration goals than has been given to Congress in the past.

They also mirror the recent change in public statements by President Reagan and other administration officials, who have moved from criticizing Nicaraguan support for Salvadoran insurgents to demanding a modification of overall Nicaraguan behavior. The United States is planning to increase its military presence in Central America sharply during the next six months, with air, sea and ground exercises, called Ahuas Tara II.

One aircraft carrier is currently operating about 100 miles off the coast of El Salvador, the Pentagon has said, and other warships are on their way.

U.N. Ambassador Jeane J. Kirkpatrick said yesterday the six-month military exercises were planned in response to a "very significant escalation" of Soviet arms and Cuban personnel in Nicaragua. "We simply feel this effort at escalation and military buildup is dangerous and unacceptable to the peace of the region," Kirkpatrick said in New York.

The commander of the U.S. Caribbean command in Key West, Rear Adm. Robert Hedges, said this week that he has not seen a significant increase in Nicaraguan arms imports from Cuba this year, and some military officials have suggested the flow may have decreased. But Kirkpatrick said the Sandinista regime in Nicaragua received 11 shiploads of heavy weapons during the first six months of 1983, compared with 14 shiploads during all of 1982.

"What caused the U.S. to decide to dispatch the task force is our concern about the continued military buildup of Soviet-bloc military strength . . . , " she said.

When the administration first requested money for a covert CIA operation in Central America in 1981, the program was described as providing training money for 500 anti-Sandinista "contras" to interrupt Cuban support through Nicaragua for the leftist guerrillas in El Salvador. The number of U.S.-supported contras has since increased to about 10,000, and the CIA is seeking funds to support 12,000 to 15,000, according to administration officials.

The covert aid program will be reviewed soon by the Senate Select Committee on Intelligence. It voted last spring not to continue funding beyond Oct. 1 unless the president explained "in plain language just what it is he wants to do relative to Nicaragua and the other countries," according to committee Chairman Barry Goldwater (R-Ariz.).

The administration's intelligence "finding," which is required by law to justify any covert operation, is intended to respond to the committee's demand for a clear rationale for the covert aid to the anti-Sandinista guerrillas. The justification may not satisfy the entire committee, however, as Vice Chairman Daniel Patrick Moynihan (D-N.Y.) suggested Sunday when he said the administration should apply pressure in Havana and Moscow rather than Managua.

In addition, the House is planning to begin voting today on a proposal that would end the covert aid to the anti-Sandinista guerrillas and authorize $80 million for an arms interdiction program to be waged outside Nicaragua. Defense Secretary Caspar W. Weinberger, in a recent letter to the ranking minority member of the House Intelligence Committee, Rep. J. Kenneth Robinson (R-Va.), said such a program could not work.

"An interdiction program which treats Nicaragua as a sanctuary would be prohibitively expensive," Weinberger wrote. He said attempting to stop the arms flow would cost $300 million the first year and at least $100 million each following year and "require extensive deployment of U.S. personnel to the area."

Weinberger was arguing only against an interdiction program that prohibits incursions into Nicaragua. But other officials said even without that restriction, indirect pressure is likely to be more successful.

As a result, an official said yesterday, the administration is "going to spell out much more clearly that the idea is to do more unto Nicaragua what Nicaragua is doing unto El Salvador."

Administration officials repeated yesterday that their goals do not include overthrowing the Sandinista regime. One top official said that no one "in his right mind" believes that 10,000 anti-Sandinista guerrillas could defeat the well-equipped Nicaraguan army of 25,000.

"On the other hand, the question is who would join in support," the official said. "The people are very unhappy. The big question is, are they unhappy enough to do something about it, and if they are, can they?"