The Reagan administration won a first-round tactical victory in Congress on El Salvador yesterday when a House subcommittee agreed to postpone voting on a resolution intended to cut off military assistance to that country.

Secretary of State Alexander M. Haig Jr. and other administration officials had lobbied members to put off the vote on grounds that it might have an adverse effect on the Salvadoran elections scheduled for March 28.

Several members said Haig had called them yesterday morning warning of the implications of a vote that could be viewed in San Salvador as a lack of confidence in the civilian-military junta the administration is supporting, and the election it is holding.

Four Republican members of a House Foreign Affairs subcommittee and a lone Democrat, Daniel A. Mica (Fla.), made up a majority favoring postponement. Four Democrats, including Chairman Michael D. Barnes (Md.), favored having a vote but Barnes was forced to call a recess. "The chairman can count," he said.

There has been growing opposition in Congress to increased military aid to El Salvador, and further U.S. involvement in Latin America generally.

Yesterday was the first time this year the aid-to-El Salvador question has approached a vote. It came as several congressmen were reported preparing further legislation that would prohibit covert U.S.-financed actions against the government of nearby Nicaragua. The Washington Post reported Wednesday that President Reagan has authorized covert operations against that Central American nation.

Rep. Jonathan B. Bingham (D-N.Y.) will introduce a bill prohibiting aid for any organization intending to conduct military or paramilitary action against the government of Nicaragua without congressional approval, an aide said.

The White House, meanwhile, continued to refuse to confirm or deny the covert-action report.

But presidential spokesman David Gergen said in response to questions, "It is not the policy of this government to topple other governments. This government does not engage in operations to topple other governments."

Haig and a succession of administration witnesses recently have implored Congress to approve stepped-up aid for El Salvador, but many members are either dubious or flatly opposed.

The confrontation yesterday was set up last year when Congress conditioned aid for El Salvador on changes in that country's attitude toward human rights. An amendment to the foreign aid bill prohibited assistance unless the president certified that there had been an improvement in human rights and more civilian control over the military forces. Reagan on Jan. 28 certified that both conditions had been met.

Rep. Gerry E. Studds (D-Mass.) and others who felt Salvadoran human rights had not improved introduced a resolution to declare that certification void.

Congressional aides said heavy administration lobbying against the Studds proposal began early this week and culminated in the Haig phone calls yesterday morning.

"The phone is ringing off the hook with calls from both State and some members," said an aide to Rep. Gus Yatron (D-Pa.).

Mica said he had decided before his call from Haig to press for a delay.

In a letter to colleagues, Mica said that a vote for Studds' resolution "could easily signal an abandonment of all U.S. interests in that country and result in increased activity and violence from guerrilla forces."

Mica also solicited support from House Speaker Thomas P. O'Neill Jr. (D-Mass.) and Majority Leader Jim Wright (D-Tex.), both of whom agreed that the vote should be put off until after the March 28 elections for a constituent assembly, he said.

Irritated by the delay, Studds said that even inaction by the subcommittee would send a signal to El Salvador, just as when Reagan "stood the facts on their head" and certified an improvement in human rights policies. Failing to act would be like giving the Salvadoran military a "blank check," he said.

Reagan almost certainly would veto the Studds proposal if it passed both houses. It could take effect only in the unlikely event that the veto was overridden.

The administration has proposed a $226 million package of military and economic assistance to El Salvador in fiscal year 1983.

Moreover, it will propose that about one-third of the new Caribbean Basin plan's $350 million and an extra $35 million in military assistance be given to that country in this fiscal year.