The timing and character of the controversy that has erupted around the fledgling presidential campaign of Sen. Joseph R. Biden Jr. (D-Del.) make the plagiarism question a serious threat to his political future, political insiders in both parties said yesterday. But most said it is premature to write him out of the race.

"They just have to try to ride out the firestorm," said Robert G. Beckel, manager of Walter F. Mondale's 1984 Democratic campaign. "There's no room for any more shoes to drop. This is not a candidacy strong enough to survive an extended crisis."

Two points were made by almost everyone interviewed by The Washington Post following Biden's news conference to deal with charges of plagiarism of campaign speeches and an incident of similar character during his law school days:In a year when voters have been sensitized to issues of personal trust and integrity by highly publicized scandals, it is politically dangerous for Biden -- or anyone else -- to be placed in a position of having to defend his personal honor and character. For a candidate who hoped to introduce himself to many voters this week under auspicious circumstances as the chairman of the Senate Judiciary Committee holding televised hearings on the nomination of Judge Robert H. Bork to the Supreme Court, having to deal with such a question is at least a distraction and perhaps a disaster.

Predictably, the Washington reaction to the sudden flap about Biden that began last Saturday was more extreme than that around the country. But several observers predicted that the national reaction could grow with media coverage of the Biden news conference. Johnny Carson in his Wednesday-night monologue joked that Biden had called his staff together after the plagiarism stories and reassured them, "You have nothing to fear but fear itself." Said one Democratic consultant, "It's the ridicule that can kill you."

For now, however, "I can't see it's much of a deal," said John Henry Anderson, the Georgia Democratic chairman and a county commissioner in Hawkinsville. "The only comment I've heard is the fellow probably ought to change speechwriters."

Bonnie Campbell, the Iowa Democratic chairman, and Larry Yatch, her counterpart in Pennsylvania, said Democrats are asking whether Biden might be the victim of "dirty tricks," engineered either by a rival Democratic campaign or possibly by Republicans seeking to embarrass a prominent critic of the Bork nomination. "There's a backlash" in Pittsburgh, Yatch said, "against the press and whoever is putting out these stories."

On the other hand, Indiana Democratic Chairman John B. Livengood said the Indianapolis News yesterday afternoon had a front-page headline, "Biden Admits Plagiarism." He judged the consequences to be serious. "I think his campaign was in trouble anyhow, with too many screw-ups," Livengood said, "and I think this could finish it off."

Labor leaders were divided in their appraisals. John Perkins, director of the AFL-CIO Committee on Political Education (COPE), said, "I don't know how it will play out in the press, but all of us know that most of the speeches we hear do not originate with the candidates." But the political director of a major AFL-CIO affiliate, requesting anonymity, said, "It illustrates something that all of us worry about with Biden, that he doesn't seem to be in control of his mouth -- or his campaign, either."

Republican strategists, looking at the developments in the other party, saw Biden as badly damaged. "The individual actions may not be that important," said Linda Divall, a GOP polling consultant, "but they smack of a certain disregard for honesty at a time when everybody is talking about honesty and morality. He's going to have to deal with a lingering credibility problem, particularly among the baby-boomers, the people for whom he's tried to make himself kind of a moral beacon."

Edward J. Rollins, manager of President Reagan's 1984 campaign and now helping guide the bid of Rep. Jack Kemp (R-N.Y.) for the nomination, said, "If his campaign were going well, this might be a minor glitch. But he's had trouble getting started, and this is going to make it that much more difficult for him to get up and going."

"I don't think anybody questions his integrity," Rollins said. "It's a question of stupidity. It's not a big character issue like you had with Gary Hart, but it makes people doubt his judgment, and anytime you put doubt in people's minds, it's hard to get rid of."

Most of Biden's Democratic rivals maintained an official silence or offered supportive public statements. Sen. Paul Simon (D-Ill.) said he had "complete confidence in his integrity," and Sen. Albert Gore Jr. (D-Tenn.) told reporters he had heard Biden give full attribution to borrowed speech material on several occasions.

In Iowa, site of the first contest next February, managers of several other Democratic campaigns speculated that it could damage Biden's chances. "Many people liked him because of his speaking," one said, "and this takes that away." But Lowell Junkins, the 1986 Democratic gubernatorial nominee and a leading Biden supporter, said the voters with whom he had talked "think this {the furor} is a big joke . . . goofier than a $3 bill."

Democratic pollster Peter D. Hart, who is not affiliated with any presidential candidate, said it was no laughing matter. "I've been one who has said 'Watch Joe Biden' because of his exceptional interpersonal skills," Hart said, "but this is the most unfortunate thing that could have befallen him.

"Most voters don't know him, and here he is on their television screens, sitting as chairman of the Judiciary Committee, talking about the standards that must be met for the Supreme Court. Suddenly they learn something that is not flattering about him. And what they learn goes right at the heart of the cardinal rule for 1988: You must play by the rules. And playing by the rules means writing your own speeches or citing the people you're quoting."Staff writers James R. Dickenson and Maralee Schwartz contributed to this report.