The United States, fearing a stepped-up effort by leftist guerrillas to disrupt the March 28 elections in El Salvador, is preparing an urgent shipment of new equipment to replace the losses suffered by government forces there in last week's guerrilla attack on Ilopango air base.

Informed sources said yesterday that the Reagan administration's immediate goal is to restore the U.S.-backed, civilian-military government's ability to provide helicopter and fighter plane support for ground operations against the guerrillas.

The extent of the damage caused in the predawn raid at the air base outside San Salvador last Wednesday has been kept secret.

But the sources said the Salvadoran government's capability to conduct aerial operations was badly impaired when guerrilla bombs destroyed or severely damaged several helicopters and jet fighter planes.

According to the sources, the Defense Department already is assembling the equipment--believed to include replacement heavy-duty Huey helicopters and spare parts--and is preparing to airlift it to El Salvador on a priority basis. The sources added that some of the equipment could be en route before the weekend is over.

Underlying this urgency, the sources said, is the administration's fear that the Ilopango attack was the opening salvo in a guerrilla attempt to derail the March elections through intensified hit-and-run attacks on El Salvador's military and economic infrastructure.

The elections scheduled by President Jose Napoleon Duarte's government are to pick a constituent assembly that is supposed to begin returning the country to civilian rule.

The electoral plan has been supported strongly by the administration, which hopes it will produce a new government capable of winning sufficient popular support to end El Salvador's long and bloody civil war.

However, a heavy infusion of emergency military aid at this time seems certain to intensify the confrontation building between the administration and congressional liberals who contend that the Duarte government continues to systematically violate human rights.

Many Democrats in Congress and human rights groups were angered when Reagan last Thursday responded to a congressional provision attached to the fiscal 1982 foreign aid bill by certifying that the Duarte government is ending rights abuses and making progress toward political and economic reform.

Two House Democrats, Tom Harkin (Iowa) and Gerry E. Studds (Mass.) already have said they will introduce legislation this week to cut off all military aid to El Salvador. The issue also is expected to trigger heated debate between liberals and conservatives when the Senate and House subcommittees on inter-American affairs holds separate hearings this week.

Although no announcements have been made yet about the emergency resupply, State Department spokesman Alan Romberg said after the air base attack that the administration was "reassessing assistance needs on an urgent basis" and added that the United States "must be prepared to increase our economic and our military assistance to El Salvador as necessary."

The sources were unable to specify the authority that the administration will use as its legal justification in the resupply effort.

As the result of Reagan's certification on human rights, the administration is free to immediately begin dispersing $26 million in military aid allotted for El Salvador in fiscal 1982.

In addition, there are provisions in the foreign aid law allowing the president to provide up to $50 million in emergency military aid to designated countries if he determines that U.S. national security is threatened. Reagan used that provision last March when the administration decided to increase substantially U.S. assistance to the Duarte government and to send American military advisers to El Salvador.