Secretary of State Alexander M. Haig Jr., speaking against a background of mounting controversy about U.S. policy in Central America, told Congress yesterday that cutting off military aid would be "a fatal blow" to the government of El Salvador in its struggle against leftist guerrillas.

"I think it would be a catastrophe," Haig said. "In practical terms, the guerrillas are not about to overwhelm the country. But American arms are a crucial factor, even more in political and psychological terms than in actual material terms."

The secretary's testimony before a Senate subcommittee came on a day that saw a rapid-fire series of developments continue to push the debate over El Salvador and Nicaragua to the forefront of congressional and executive branch attention. Among events yesterday and last night were these:

* Twenty-six prominent officials of past Republican and Democratic administrations emerged from a classified intelligence briefing at the State Department to say they had been given convincing evidence that the Salvadoran guerrillas are being aided, supplied and guided from outside by Cuba and Nicaragua. However, the former officials also agreed that the evidence is so "sensitive" that it cannot be made public without causing serious damage to U.S. ability to collect intelligence in the future.

* Haig, Defense Secretary Caspar W. Weinberger and some other senior administration officials refused to comment on a report yesterday in The Washington Post that President Reagan has approved a $19 million program to destabilize the revolutionary Sandinista-dominated government in Nicaragua. Instead, they took the position, described by Haig as "consistent and longstanding policy," that the government does not comment publicly on covert activities, but they added that their silence should not be interpreted as confirmation of the report.

However, White House counselor Edwin Meese III told a journalism seminar here last night that The Post report placed the administration in a "totally untenable position." Meese said: "Here is a situation in which the security of the country is seriously affected, whether the article be true or false. If it is true, then very important secrets vital to our national interest and vital to the success of any such mission have been revealed. If it is false, then you have given an adversary a great opportunity for propaganda . . . . "

* Haig said the administration has not rejected Mexican President Jose Lopez Portillo's offer to act as a mediator in trying to end the Salvadoran civil war and ease the tensions between the United States and Nicaragua. The secretary said he would continue discussing the idea with Mexican Foreign Minister Jorge Castaneda in New York this weekend; in another hint that Washington wants to keep alive the possibility of eventual negotiations, Haig expressed hope that after the March 28 elections for a constituent assembly in El Salvador, the guerrillas might be induced to lay down their arms and negotiate "in the context of a plebiscite that would express the will of the Salvadoran people."

* Weinberger and Gen. David C. Jones, chairman of the Joint Chiefs of Staff, reiterated, in testimony before the House Foreign Affairs Committee, that the administration has no plans or intentions to send U.S. troops to El Salvador and will confine its support of the military-civilian government there to increased military and economic aid.

The administration, obviously concerned that its policy toward Central America could be derailed by persistent questions about whether the United States is heading for a new Vietnam-type involvement, this week mounted a campaign to swing public opinion behind its approach.

The effort began Tuesday when senior intelligence officials gave reporters a detailed briefing, including the display of blowups of aerial reconnaissance photographs, designed to show that Nicaragua is building a sizable military establishment with Cuban and Soviet help. The second stage came yesterday with the briefing aimed at enlisting the support of former officials and with Haig's appearance before a Senate Appropriations subcommittee that oversees foreign aid.

Under heavy questioning from Republican and Democratic members, Haig reiterated his past charges that the Salvadoran guerrillas are "largely controlled" from the Nicaraguan capital of Managua. He said the principal flaw in the Mexican peace initiative, which he asserted is basically the same plan proposed by the United States and rejected by Nicaragua last year, is the lack of a requirement that Nicaragua "commit itself to cease and desist" in its involvement with its neighbors.

"We are exploring every feasible means possible to bring about a peaceful solution," he said. "But this is a two-way street, and it requires reciprocal obligations by those who brought about this crisis in the first place."

The U.S. position has been to back the Salvadoran junta's call for the guerrillas to stop fighting and take part in elections designed to determine the country's future. However, Haig and other administration officials have emphasized repeatedly that they are firmly opposed to negotiations that would mean a division of power allowing the guerrillas to "win at the negotiating table what they have failed to win on the battlefield."

When viewed in that context, Haig's comments yesterday about the possibility of future negotiations did not depart openly from stated policy. But, there was a hint in his remarks that the administration, aware of the possibility that the upcoming Salvadoran elections might produce a victory for the extreme right or some other unacceptable result, wants to keep alive the option of a new try at negotiations through Lopez Portillo or some other means.

He conceded that the March 28 elections "are not likely to solve the problems of El Salvador and that it will be necessary to move, as fast as possible, to the next stage of writing a constitution and arranging for the election of a president."

Haig noted that "there are many genuinely democratic elements" in the leftist front opposing the goverment, but warned that "it is the hard-core, armed, professionally trained guerrillas who will ultimately take over if all things remain equal." Should the left, including its guerrilla elements, rid itself of outside influence, agree to a cease-fire and join in negotiations on the next electoral stage, he emphasized, "we would be very much in favor of it."

The administration's other main effort yesterday involved the briefing, presided over by Haig and Central Intelligence Agency Director William J. Casey, for former officials and members of the president's Foreign Intelligence Advisory Board. Following the meeting, Anne Armstrong, chairman of the advisory board, denied that the group consisted solely of people already committed to support of the administration's policy.

She characterized them as "a bipartisan group of patriotic Americans" and said she believed all present had found the administration's evidence convincing. Asserting that the information is too sensitive to be made public, she added: "I wish it were not so because it's a story that desperately needs to be told to the American people."

Her comments were echoed by several of the others including former secretary of state William P. Rogers, former national security affairs advisers Brent Scowcroft and Zbigniew Brzezinski and former Democratic special ambassadors Sol Linowitz and Robert Strauss. All described what they had heard with such terms as "convincing" and "disturbing" and said they felt it had to be kept secret if the United States is to continue receiving accurate intelligence on Central America.

The administration's refusal to discuss The Washington Post report on alleged covert action plans did not draw much immediate reaction on Capitol Hill. However, Rep. Michael D. Barnes (D-Md.), chairman of the House inter-American affairs subcommittee and a critic of administration policy, said the plan, if true, amounts to "a virtual declaration of war."