The Senate Foreign Relations Committee dealt the Reagan administration its biggest foreign aid defeat yesterday by voting unanimously to reduce the 1983 aid requst for El Salvador by $100 million.

The vote reflected disillusionment with the Salvadoran government's handling of land reform, which critics claim is grinding to a halt under the country's newly elected leadership.

A few hours later, a House subcommittee added another setback by rejecting the entire $301.5 million military aid supplemental package for this year, including $35 million for El Salvador.

The double blows were the first major foreign aid requests lost by the administration, and left in a shambles its plan for a dramatic increase in security assistance to the Central American country it has chosen as the key to blocking communist insurgency in that region.

The rebuffs show that congressional opposition to aiding El Salvador is increasing despite elections there in March that were hailed by the State Department as a milestone in developing democracy.

The turning point was the recent action by the right-wing dominated Salvadoran Constituent Assembly that appeared to suspend part of a key land-reform program that had guaranteed thousands of peasants ownership of small farms for raising grain and cattle.

Assistant Secretary of State Thomas O. Enders tried to deflect Senate opposition by asserting that the intent of the assembly's action is not clear and promising a full analysis next month when President Reagan is required to ceritfy Salvadoran eligibility for continued aid.

But several senators claimed that evidence shows a slowdown in land reform. They pointed to increased evictions of tenant farmers and a virtual cessation in granting provisional land titles since right-wing parties took control of the government after the election.

"It should be clear where they are headed on land reform," said Sen. Christopher J. Dodd (D-Conn.), who introduced the amendment cutting back Salvadoran aid. "They are out to destroy it."

The amendment ultimately sponsored by Dodd and Sen. Nancy Landon Kassebaum (R-Kan.) freezes security assistance for El Salvador in fiscal 1983 at the 1982 level of $66 million.

The administration had asked for $166.3 million in 1983 for that package, which includes military aid and economic support funds associated with security. Regular economic development money requested by the administration would not be affected.

The amendment also would prohibit all aid to El Salvador if the government there alters, suspends or terminates the land-reform decree issued in 1980 by the former government of Jose Napoleon Duarte.

Several members referred to the action as a "signal" sent to Salvadoran leaders in hopes of reversing the direction on land reform. The authorization bill in which it is included could be changed later on the Senate floor or in conference with the House.

But in the current mood of Congress, even normal economic aid also may be cut off. Reagan must formally certify in late July that El Salvador is making progress on land reform and improving human rights in order that aid continue.

Enders told the committee that the administration agrees that land reform must go forward to justify continuing aid. He said the key questions are whether the government attempts to control tenant evictions and starts to issue provisional land titles to landless peasants.

Kassebaum, Dodd and other committee members have long contended that land reform is the key to attracting Salvadorans to the central government and discouraging them from joining insurgent forces.

The House Appropriations subcommittee on foreign operations voted, 7 to 5, to reject the administration's entire 1982 foreign military aid supplemental request that included $35 million for El Salvador and substantial amounts for South Korea, Turkey, Spain, Thailand and other countries.

Chairman Clarence D. Long (D-Md.) called the administration's request part of a "game" to obtain funds that Congress refused last year in passing the first foreign aid money bill in three years. "I'm not going to be part of that game," he said.

Yesterday's actions left in doubt the administration's other major foreign aid project, the Caribbean Basin initiative. Republicans on the House committee asked that consideration of it be deferred, apparently fearing that it too might be dismantled. It includes $128 million for El Salvador.

The Senate committee already has transformed the Caribbean plan into a multilateral aid program to be run by the World Bank.