Secretary of State Alexander M. Haig Jr., joking about "Haig Must Go" headlines, disarmed some conservative critics yesterday with a skillful blend of wit, hard-line rhetoric and diplomacy.

Venturing before what was expected to be a hostile audience, Haig the politician and stump speaker took over from Haig the controversial diplomat.

"I know my conservative credentials have been challenged," he began, but when he finished 90 minutes later Haig left a crowd of more than 400 on their feet cheering at the Conservative Political Action Conference at the Mayflower Hotel.

He did so by interlacing a defense of the Reagan administration's foreign policy with some self-deprecating humor. He called his speech, for example, "the first annual Bob Woodward address on confidentiality in government," a reference to leaked notes of Haig staff meetings that appeared in The Washington Post.

"I talked about this speech with my aides yesterday, so most of you have probably read about it already," he added.

Haig said that he had decided, in the spirit of President Reagan's "New Federalism," to delegate part of the nation's foreign policy to the governors of the 50 states. "That will reduce by at least one-half the number of foreign policy spokesmen," he joked.

Haig's appearance came at a time of mounting conservative criticism of American foreign policy, and he was clearly trying to re-establish his credibility among Reagan's conservative supporters. He was scheduled to appear for 20 minutes, but spoke 90 minutes and didn't leave until he had fielded a host of questions on such favorite conservative topics as the Council on Foreign Relations and the Trilateral Commission.

Haig defended both. The Council of Foreign Relations "has been reflective of the Northeast establishment," he said, but "I think you'd be hard pressed to call it subversive." In Europe, he said, the Trilateral Commission is made up of hard-line conservatives.

Some conservative leaders were both impressed and dismayed on how Haig dispelled their criticism. "He left a feeling of good will for himself. He defused a lot of ill will," said John Lofton, editor of Conservative Digest, which has attacked Haig frequently. "He gave a good performance. Of course, I didn't expect him to come in here and resign."

Last month Human Events, a conservative newspaper, called for Haig's ouster in an article headlined "Haig Must Go." An informal survey taken by the Conservative Digest at the conference Friday found him to be the least popular member of the Cabinet: 51 percent of those polled said Haig should resign.

The chief conservative complaint against Haig has been that he surrounded himself with "non-Reaganites" and "Kissinger retreads." He also has been accused of vacillation on policy toward Poland, the Soviet Union and Central America.

Only hours before Haig spoke, for example, Sen. Jesse Helms (R-N.C.), the spiritual leader of New Right conservatives, charged that "Kissinger retreads" in the State Department led to the situation in El Salvador.

"When the Reagan administration took over there was a forthright recognition that the arms supply for the revolutionary movement was coming through Nicaragua and Cuba," Helms said. "Recognition of the problem almost stopped there.

"Much of the Carter policy apparatus in Central America stayed in place," he added.

Helms drew wide applause, drawing from some of the oldest topics in the conservative arsenal: attacking the mass media, the Panama Canal treaties, communism and Salt II. "The major media of this country are trying to do to El Salvador in six months what they did to Vietnam in six years," he said. "They are trying to convince us that a negotiated settlement with Marxist terrorists can bring peace and security."

Haig also had some harsh words on this subject. He said that "Salvador is not Vietnam," and insisted that Americans will support the efforts of the U.S.-backed junta there against leftist subversion "provided they believe we mean business, and we're going to succeed."

But by comparison with Helms, Haig sounded moderate, defending Reagan's record defense budget against attacks "from the left and the right."

He repeatedly cautioned conservatives that "history didn't begin one year ago," and that "socio-economic" factors play a critical part in every foreign policy decision.

He conceded that he had been accused of "being the soft guy" for ignoring calls for unilateral American embargoes against the Soviet Union and not calling for a default on the massive Polish loans to the United States.

These options did not stand up to careful analysis, he said. "They make us feel good, but all would collaspe because of lack of popular consensus."

Last night the sponsors of the conference, the American Conservative Union and Young Americans for Freedom, presented a special award to Zdzislaw Rurarz, the former Polish ambassador to Japan who renounced his post in protest against martial law in Poland.