Librarian of Congress Daniel J. Boorstin came under sharp attack in a congressional hearing yesterday for cutting by one third the hours the library's reading rooms are open to the public.

"Why?" demanded Rep. Newt Gingrich (R-Ga.). " . . . It is a 35 percent cut!"

Earlier Boorstin had testified that, to meet congressionally mandated budget cuts, "we made a cut of only 2.8 percent in our expense for reader services, as against a 22 percent cut in acquisitions." Reader services includes the public reading rooms, the site of recent sit-ins protesting curtailed hours.

But Gingrich said he had just performed a few calculations on the spot. "For the consumer, there's a 35 percent cut."

Replied Boorstin: "The outcry of people about reading room hours is an outcry of one very small part of our constituency." He said the shorter hours are "a small sacrifice to ask" to prevent more extensive damage to the library's acquisition program.

"We are not a public library. We are a national library," Boorstin said as the argument heated. " . . . Our job is to serve the Congress and the nation." He recommended that people use their public libraries.

Cutting the reading room hours, Boorstin testified, had saved $585,000.

Rep. Bill Frenzel (R-Minn.) joined the fray, saying, "We think there's an important priority in making what's in that library available not just to scholars, but to whoever walks in off the street."

He asked how much the library was spending on its American Folklife Center.

Answer: $850,000.

Frenzel suggested that this and other "nonlibrary services" might be cut.

Boorstin answered that if he got more money he'd "think twice" before putting reading rooms ahead of acquisitions. "It would be unfair if the living generation did not share to this small extent in our common sacrifice," he testified.

But why, pressed Rep. Al Swift (D-Wash.), that 35 percent reading room cut when reader services as a whole were cut only 2.8 percent?

John C. Broderick, whose research services division includes the reading rooms, explained that "there are very few controllable costs in my department . . . There is almost no other way to do it."

"Life is tough all over," said Swift, referring to the squeeze on government agencies under the Gramm-Rudman-Hollings budget reduction act. "The FBI is cut in half in my district."

Rep. Frank Annunzio (D-Ill.), who chaired yesterday's hearing by the Joint Committee on the Library and who repeatedly called the Librarian of Congress "Dr. Bornstein," said he supports the library.

He added, "I want the record to show that everybody is tightening his belt, and it takes guts." Budget cuts of $18.3 million this year forced library officials to cut public hours, reduce staff and curtail expenses for acquisitions, preservation, services for the blind and handicapped and other activities.

On March 10 the public reading rooms, which had been open weekdays until 9:30 p.m., began closing at 5:30 p.m. except on Wednesdays, when they remain open until 9. Sunday hours were eliminated.

Boorstin and other top library officials have refused requests for interviews to discuss the budget cuts.

There was also testimony yesterday from librarians, scholars and members of the public.

Fredric Alan Maxwell, who belongs to Books Not Bombs, the group that has staged sit-ins at the reading rooms to protest the curtailments, said in his prepared statement: "Mahatma Gandhi was once asked what he thought of western civilization. Gandhi replied, 'I think it would be a good idea.' "