The Senate Intelligence Committee unanimously welcomed and approved the nomination of John N. McMahon as deputy director of the CIA yesterday after one member cautioned that some lawmakers still do not have full confidence in CIA Director William J. Casey and thus were relying especially on the new deputy "to be straight with us."

At the same time, McMahon sought to assure the committee that new presidential orders governing CIA operations did not mean that the agency would be involved in so-called "intrusive" operations in this country involving U.S. citizens.

Asked by Sen. Walter D. Huddleston (D-Ky.) whether the panel would be informed whenever such techniques "are being used against Americans at home," McMahon said:

"I don't think the CIA will ever be involved in intrusive techniques against Americans here in the United States. Should there be such a requirement, the FBI would do that and probably with a court warrant."

McMahon's answer seemed to go beyond a prepared opening statement to the committee in which he said:

"I would like to emphasize for the record that the activities of the intelligence community involving Americans are, and must continue to be, limited, subject to strict standards of accountability and far removed from any abridgement of cherished constitutional rights."

Huddleston and others have charged that the language of the executive order signed by President Reagan last December does widen CIA authority to operate in the United States rather than strictly overseas. The intrusive techniques referred to usually mean such things as wiretapping, mail opening and searches without a warrant.

McMahon, 52, was praised by all committee members for expertise and professionalism during a 31-year CIA career. But several senators expressed the view that McMahon faced an especially "heavy burden," as Joseph R. Biden Jr. (D-Del.) put it, as successor to retiring Adm. Bobby R. Inman.

Inman's surprise decision to resign, revealed last month, shocked the panel. Committee Chairman Barry Goldwater (R-Ariz.) yesterday called Inman "the best intelligence officer I have known," a compliment echoed by many members.

But Biden carried the point much further. Among Inman's other skills, he pointed out, the admiral "had a heck of a relationship with this committee."

But " . . . Some of us at least," Biden continued, "don't always leap to embrace the utterances of Mr. Casey as being the whole story. That may be a little unfair, and let's assume it is." But the fact remains, Biden added, that " . . . We sometimes wonder whether we're getting the whole truth" from Casey "or whether it's politicized."

Biden said that he and others could always count on Inman for the full story, and he and Goldwater joked that McMahon ought to learn how to pull up his socks or slide back his chair at the witness table, as Inman reportedly did on hearing other witnesses say things that troubled him.

Allegations about Casey's "politicizing" of intelligence are not new. Yet, paradoxically, Inman is known to be one of Casey's strongest defenders in terms of the director's rejection of any attempts to manipulate intelligence information.

Unlike Inman, widely regarded as an "idea man" with a good grasp of global strategy, history, politics and technical matters, McMahon is seen as strong mainly in management and technical fields. McMahon also is credited by top CIA insiders as being the most resistant to any form of outside manipulation of intelligence.

In his statement McMahon pledged allegiance to the benefits of congressional oversight of secret CIA activities and, under questioning, promised to inform the committee if he learned that important information had been withheld or if the panel had been misled or misinformed.

He also said the CIA in July will complete a new study, ordered by the White House, to assess U.S. counterintelligence capabilities for dealing with the threat posed by foreign agents.

In a related development, the committee's former chairman, Democrat Frank Church of Idaho, warned yesterday that "there is every evidence" that the United States is losing sight of earlier guidelines and the general proposition that covert operations "should be a rare occurence."

"If we are not careful," he warned at a conference sponsored here by the Campaign for Political Rights, "we will return to past practice in which covert operations become a routine program involving literally hundreds of projects each year in dozens of countries.

"We will find once again that these projects, taken in the aggregate, can have powerful and adverse consequences."