Edward A. Curran, the Reagan administration's controversial nominee for the chairmanship of the National Endowment for the Humanities, yesterday found his credibility, commitment and qualifications under question in confirmation hearings for a job often described as "the nation's leading humanist."

Curran, a former headmaster of the National Cathedral School for Girls and present deputy director of the Peace Corps, made headlines in 1982 when he wrote to President Reagan suggesting the abolition of the National Institute of Education. Curran was director of the institute at the time, and his letter, which criticized the federal agency as a tool of the political "left," came just months after he had assured the same Senate confirmation panel that he would do his best to promote the institute's work.

It was that seeming inconsistency, as well as his stormy tenure at the Department of Education and the Peace Corps, that prompted a lineup of senators, including Claiborne Pell (D-R.I.), Edward M. Kennedy (D-Mass.), Christopher Dodd (D-Conn.), Lowell P. Weicker (R-Conn.), Paul Simon (D-Ill.) and John Kerry (D-Mass.), to question Curran closely and at times harshly about his plans for the humanities endowment.

"There's a saying that goes 'Once shame on you, twice shame on me,' " Weicker told Curran. "What's at issue . . . is testimony before this very committee that is clearly at odds with subsequent events. And now here we are again, and you've made statements supportive of the National Endowment, but . . . God knows if you won't get in there and find more members of the 'left' and decide that abolishing the agency is the way to deal with it."

The hearing appeared to raise as many questions as it answered, and is likely to prolong the controversy over a nomination that has been stalled for months. The NEH distributes more than $100 million in federal money each year for scholarly research and has been a leader in the effort to revive humanities studies in the nation's schools.

Curran, who downed 10 glasses of water during the four-hour hearing, told the members of the Senate Labor and Human Resources Committee that he had learned to work through channels since sending the Reagan letter, and pledged to "give my utmost" to ensure that the endowment "is a visible leader in supporting and encouraging the best of America's effort in the humanities."

Curran's supporters, who include committee Chairman Orrin G. Hatch (R-Utah), have maintained that Curran's career as a secondary school administrator makes him the ideal man to lead the endowment in its quest to increase the role of the humanities in American high schools and universities. They have portrayed opposition to the nomination as elitist.

Curran's critics have pointed out that he lacks a PhD. Yesterday's objections, however, appeared to have less to do with Curran's relatively few academic credentials (he has an MAT from Duke University) than his limited experience with scholarly research or writing and his record at the Department of Education and the Peace Corps.

Curran was forced to resign his position at the National Institute of Education after he clashed with his boss, then-Education Secretary Terrel Bell, over the letter to Reagan.

At the Peace Corps, Curran has crossed swords with his boss, Director Loret Ruppe, on numerous occasions. Their dispute has been described as personal and ideological, and reached a nadir last year when Ruppe was discovered to have secretly tape-recorded a conversation she had with Curran about the continuing rancor in their relationship. Since then, Ruppe reportedly has given Curran few key assignments and excluded him from most key meetings.

Curran's responses to questioning yesterday seemed to do little to reassure the scholars testifying and in the audience. "The hearing was disturbing," said O.B. Hardison, chairman of the board of the Washington-based Humanities Alliance, an association that represents more than 100,000 humanities scholars. "We feel that some grave questions were raised, and our concern is deepening."

Senators Pell and Kennedy said they had received more than 150 letters between them from constituents and humanities groups concerned about Curran's nomination. Pell said it was difficult to recruit academics to testify against Curran's nomination, because they fear loss of NEH money should he be confirmed.

The White House named Curran nine months ago to succeed William Bennett, who left the chairmanship to became secretary of education. Since then Curran's nomination has languished in the Senate, reportedly stalled by congressional moderates and humanities groups.

Curran, 52, became headmaster at the National Cathedral School in 1968 after working as a teacher at a private boys' school in Houston, where he got to know Vice President George Bush. In 1980 he worked as a volunteer on the Reagan-Bush campaign before becoming associate director of presidential personnel at the White House.

Curran was accompanied yesterday by his wife, Nancy duPont Curran, and an aunt, and remained in the committee room for the entire four-hour hearing, sitting without expression as two influential academics testified that they could not support his nomination.

"There are lots of distinguished humanists who have no PhDs," said Theodore J. Ziolkowski, president of the Modern Language Association in Princeton, "but we wouldn't worry about them because we know their values from their ((SECTION)cholarly work."

William Schaefer, executive vice chancellor of UCLA, echoed those sentiments. "I believe the presidential nomination is insensitive and inappropriate. No matter how any one of my colleagues might have felt about Joe Duffy or Ron Berman NEH chairmen under presidents Carter, Nixon and Ford , both had a record as outstanding teachers and outstanding scholars, and because of that the endowment was accepted and nourished by the scholarly community."

It was unclear at the close of the hearings when Curran's nomination will come to a vote.