An emotional Sen. Joseph R. Biden Jr. (D-Del.) yesterday acknowledged he had plagiarized in a paper he submitted while a first-year law student in 1965, but defended his integrity and vowed to remain a candidate for his party's presidential nomination.

"I did something very stupid 23 years ago," Biden, chairman of the Senate Judiciary Committee, said at a crowded news conference he convened to try to dispose of burgeoning charges of plagiarism -- past and present -- that have threatened his candidacy.

He said his 1965 "mistake" was neither intentional nor "malevolent," noted that the faculty of the Syracuse University Law School had allowed him to repeat the course -- after initially flunking him for lifting without citation five pages from a published law review, and said that his dean later vouched for his high character. "If anyone tells you Joe Biden isn't a straight arrow, I'd be very surprised," Biden said.

He also dismissed as "much ado about nothing" charges that in his stump speeches this year, he has used -- word for word and without attribution -- passages from other politicians.

Biden said he rarely failed to attribute and that these "mistakes" were born either of inadvertency or ignorance. He added, "In the marketplace of ideas in the political realm, the notion that for every thought or idea you have to go back and find and attribute to someone is frankly ludicrous."

The flap unfolded at the worst possible time for Biden, throwing his campaign into crisis just as he began introducing himself to the nation as chairman of the committee conducting the televised hearings on confirmation of Supreme Court nominee Robert H. Bork.

He said the timing of the leaks to the news media was "no coincidence" but declined to point fingers at any specific opponent. Meantime, the political community was busy yesterday playing a kind of "whodunit," trying to figure out which of Biden's Democratic rivals or which Republican interested in discrediting him in the midst of the hearings was the source of the leaks.

Biden, 44, has never been bashful about showing emotion. And his demeanor at the 35-minute news conference shifted from contrition and defensiveness to take-me-as-I-am defiance. "I've done some dumb things, and I'll do some dumb things again," he said at the beginning. He ended it: "I'm in this race to stay. I'm in this race to win. And here I come."

Biden's supporters said afterward that they think he contained the damage. "Are we only going to allow people who have never made a mistake to run for president?" asked Lowell Junkins, a 1986 Democratic candidate for governor in Iowa and cochairman of Biden's campaign in that key state.

"We've all flunked a test," he continued. "We've all failed to be above average on occasions. I think Amercia is going to relate to Joe Biden because he's an average person . . . . Being an average person means that sometimes you do things below average, and you sometimes do things above average."

"We're battle-scarred, battle-tested and ready to do battle," said Biden's press secretary, Larry Rasky. "His deepest dark secret is out, and it was a term paper in his first semester in law school and it ain't a big deal."

While describing the law school incident, Biden released a stack of documents that contained correspondence on the episode, as well as law school and undergraduate transcripts.

The record showed that in a meeting on Dec. 1, 1965, the law school faculty found that Biden had, "without quotation or citation," lifted five pages from a published law review article and used them in his 15-page paper for a legal-methods course.

The faculty recommended that he receive an "F" for the course and be allowed to repeat it the following year. (Biden did repeat, receiving an 80). Law School Dean Ralph Kharas said in the memo that if Biden's record was clear from that point on, he would state that the incident should not stand in his way to his being admitted to the bar. Three years later, Kharas' successor, Robert W. Miller, wrote a letter to the Delaware Board of Bar Examiners stating that Biden's records "reflect nothing whatsoever of a derogatory nature."

But as the incident was initially being adjudicated by the faculty in 1965, Biden clearly felt in danger of expulsion. In a letter to the dean and faculty, he explained he did not think it was "possible to plagiarize" the legal memorandum because he was "under the misguided understanding" that the sole purpose of the assignment was to demonstrate an understanding of the form of legal writing and provide a critic with source materials to consider.

He concluded his appeal: "I am aware that, in many instances, ignorance of the law is no excuse. Consequently, if you decide that this is such an instance and that I've broken the law, then any course of action on your part is justified. But please, I implore you, don't take my honor. If your decision is that I may not remain at Syracuse University College of Law, please allow me to resign, but don't label me a cheat."

Biden's instructor in the legal methods course, David Yaffee, recalled yesterday that he had been "disturbed" by the incident and that he had never had a similar one in five or six years of teaching the course. But he added: "I was not unhappy with the outcome of the faculty meeting. They felt he was a very foolish young man, and he was scared silly."

Biden said jokingly yesterday that his greatest embarrassment was that his sons were going to find out the mediocrity of his academic record. He said he finished 76th in a class of 86 or 87 at Syrcause, adding, "I hated law school."

As a candidtate, Biden has prided himself more on his ability to make an emotional connection with voters than for his skill at detailed discussions of public policy. Indeed, he has often ridiculed the idea that anyone can be elected president on the strength of position papers.

Biden's critics over the years have accused him of being a lightweight -- all flash and no substance. He was asked yesterday whether the more recent charges of lifting campaign passages from other candidates showed a pattern of sloppiness, an inability to police his words. "I feel very capable of using my mouth in sync with my mind," he said.

Biden did acknowledge a series of mistakes:He said he failed to attribute to the late Sen. Robert F. Kennedy a lengthy passage about each generation's opportunity to "bend history" in a speech he gave to the California Democratic Party this year. "I thought it was a piece of brilliant work by my staff. . . . I did not know that was an RFK quote. I should have," Biden said. He said he makes a practice of giving an appropriate citation whenever he quotes a lengthy passage from British Labor Leader Neil Kinnock about the strength of his ancestors and that he has failed to do so only once, in his closing remarks in a televised debate in Iowa last month. Later, Rasky acknowedged that Biden also failed to cite Kinnock when he used the passage in a subsequent taped presentation to the National Education Association. He attributed to "extra-exuberance" his false claim, made to audiences on the campaign trail, that a tape recording of the Kinnock speech had been given to him by former British minister Denis Healey. In fact, he said, it was given to him by William Schneider, a political analyst at the American Enterprise Institute.

At the the news conference, Biden was asked whether there was something "borrowed" not just in his specific words but in the overall rhetoric of a candidate whose speeches evoke the idealism of the 1960s but who did not participate in the social movements of that era.

"During the 1960s, I was, in fact, very concerned about the civil rights movement," Biden said, in a rambling, biographical answer. "I was not an activist. I worked at an all-black swimming pool on the east side of Wilmington . . . . I was involved, but I was not out marching . . . . I was a suburbanite kid who got a dose of what was happening to black Americans . . . .

He added, "by the time the war movement was at its peak, when I was at Syracuse, I was married, I was in law school. I wore sports coats . . . . I'm not big on flak jackets and tie-dyed shirts and -- you know, that's not me."

Even before the plagiarism charges began circulating, Biden's campaign had been viewed as one that had been unable to capitalize on its considerable potential. He has been near the back of the Democratic pack in early polls, and his campaign has been plagued by changes of message and staff.

Meantime, staffers in other Democratic campaigns spent yesterday denying a role in leaking the charges to the news media and denouncing whoever did.

"Whoever took elaborate pains to alert the press to this incident," Sen. Albert Gore Jr. (D-Tenn.), a presidential candidate, said of a videotape that juxtaposed the Biden and Kinnock speeches and was delivered late last week to selected members of the press, "apparently placed a much higher priority on damaging Joe Biden than on ensuring that the nation had a full opportunity to explore, without distraction, Judge Bork's qualifications."Staff writers Ruth Marcus and Bill McAllister contributed to this report.