Secretary of State Alexander M. Haig Jr. yesterday denied vehemently that the Reagan administration would support a right-wing military coup in El Salvador, and warned that such a move could have serious consequences for continued U.S. support of the government there.

Haig's comments were part of a strong and concerted administration effort to deny statements made Tuesday in El Salvador by a former Salvadoran intelligence officer, Maj. Robert D'Aubisson, who said his contacts with Reagan administration officials had convinced him that they would not object to a coup.

After a foreign policy briefing for the Senate, Haig told reporters:

"I want to emphasize that this does not represent U.S. policy in any way. It is counter to the policy we have been implementing and pursuing in El Salvador, and such an outcome would have serious consequences for our ability to continue to pursue those policies."

Among Reagan administration officials with whom D'Aubisson said he had unspecified meetings was Roger Fontaine, now Latin American affairs specialist on the National Security Council staff. Haig and Fontaine's last meeting with D'Aubisson took place more than a year before President Reagan assumed office.

At the White House, press secretary James S. Brady said, "Roger Fontaine has authorized us to say the statements made by the major regarding his views on the Salvadoran government are pure fiction. He said that he never stated or implied anything of the sort to anyone, anywhere, at any time, categorically denying the major's claim."

Yet another denial came from State Department spokesman William Dyess, who said, "There is nothing the president has said, the secretary of state has said, publicly or privately, or any official of this government has said that would warrant reaching that [D'Aubisson's] conclusion."

Dyess did raise eyebrows reporters at the State Department daily briefing when he refused to reply directly to questions about whether the United States would oppose a coup. While saying that the United States is opposed in principle to military takeovers, he insisted that his answer should not be applied to "any particular situation."

The administration has become increasingly sensitive to suggestions that it views the Salvadoran civil war primarily in terms of the East-West conflict and that its zeal in combating communist-bloc support for Salvadoran leftist insurgents is greater than its concern about the excesses of the extreme rightist factions in that conflict.

During recent days, the administration has increased significantly its efforts to assure Congress, the American public and foreign governments that it supports democratic reform in El Salvador.

The administration also was defended the civilian-military government headed by President Jose Napoleon Duarte, a Christian Democrat, against charges that it is a captive of the right, contending that Washington considers the government to be centrist and reformist.

Despite these assurances, an undertone of skepticism and nervousness has been evident in Congress, especially since Monday, when the administration announced that it is sending 20 additional U.S. military advisers ad $25 million in new military aid to bat the guerrillas.

However, the concern on Capital Hill -- centered on comparisons with perceptions that Washington is backing a government with no popular support -- does not seem to be translating into any serious move to oppose administration policy.

Congressional sources said yesterday that most members of Congress appear reluctant to challenge the administration too vocally. Inhibiting them, the sources said, are such factors as the administration's success in portraying the Salvadoran guerrilla movement as communist-supported and the desire of Republicans, who control the Senate, to maintain a loyalist posture.

As one source noted, "Even on the Democratic side, there's a clear tendency to be tigers when they have Haig or some other officials, in a closed committee hearing and then turn into pussycats when they're out in public."

In the Democratic-controlled House, those members who could be called a hard-core bloc of opposition to the Salvadoran policy number only about 50.

One, Rep. Richard L. Ottinger (D-N.Y.), yesterday challenged the administration's contention that the War Powers Act does not apply to its dispatch of advisers to El Salvador by introducing a resolution calling on Reagan formally to Congress on his actions.

In the Senate, criticism of the administration has been even more muted, although two Democratic members of the Foreign Relations Committee, John Glenn (Ohio) and Alan Cranston (Calif.), have shown signs of becoming potential point men for a possible offensive against the policy.

So far, the sources said, they and other Senate critics are relatively isolated. But, the sources added, if the U.S. military involvement in El Salvador should increase, Glenn and Cranston could be the nucleus of a movement that could bring the committee's other Democrats into open support of their position.