Late last fall, Sen. Joseph R. Biden Jr. (D-Del.) decided to rip into the Reagan administration as being too soft on white-collar crime. He lambasted the Justice Department's handling of controversial cases, from E.F. Hutton to General Dynamics, and prodded the Senate Judiciary Committee into announcing an ambitious schedule of weekly hearings.

When the first hearing began, however, the Delaware Democrat was gone before the leadoff witness finished his opening statement. Biden's enthusiasm had seemed to wane. The next four sessions went largely unnoticed; the unfinished hearings have been put on ice.

Biden planned another assault last fall, this time against President Reagan's judicial nominees. Since then, Biden and his allies have defeated one nominee, Jefferson B. Sessions III, come within a vote of blocking confirmation of another, Daniel A. Manion, and turned the quality of Reagan's judges into a page-one controversy.

The two efforts illustrate the best and the worst of Joe Biden as he makes preparations for an increasingly likely campaign for the Democratic nomination for the presidency. He is a 44-year-old senator with exceptional political instincts and a questionable attention span, an agile mind coupled with a restless ambition.

One of the Democratic Party's most powerful speakers, Biden's rhetorical flights in the Senate occasionally veer off course. His blunt manner and disarming candor have won over most of his colleagues, even those who say he hasn't reached his potential.

"Joe is capable of being a far better senator than he is," said a Democratic senator who says he admires Biden. "He's a quick learner, but he doesn't have the tiger instinct. He wants to be all things to all people. He needs to pick more tough fights and be willing to lose a few."

But a frequent Republican adversary, Sen. Orrin G. Hatch (R-Utah), said, "Joe would be the best candidate the Democrats could put up in 1988. He's very bright, very articulate. He's tough in the clinches. He's done some very good things on the Judiciary Committee. His major defect is that he goes on and on and on."

Sen. William S. Cohen (R-Maine), who lifts weights with Biden in the Senate gym, calls him "a good athlete who doesn't spend as much time at it as he should" -- a comment that some might view as a metaphor for Biden's 14-year Senate career.

To be sure, many of Biden's legislative accomplishments have gone unnoticed by the national news media. He played a key role in a sweeping 1984 rewrite of the federal criminal code. He helped negotiate the compromise that saved the Civil Rights Commission. He was one of the Democrats' sharpest interrogators during the 1979 hearings on the SALT II treaty. Key Role in Rewriting Criminal Code

When Biden focuses on an issue, he can go toe-to-toe with anyone. His fiery assault on Secretary of State George P. Shultz -- for defending a South African policy that Biden said lacks "moral backbone" -- was replayed on every network news show last week. Hours later he was leading the Democratic floor fight against the Manion nomination.

Yet Biden has never carved out a piece of Senate turf. "I think Joe has a low threshold for just dealing with one issue," Cohen said. "He's more thematic. He's not going to sit there and grind it out night after night, going through some detailed report."

"If you're picking out the 10 senators who have shaken the institution, you wouldn't pick Joe Biden," said political scientist Norman Ornstein of the American Enterprise Institute. "Biden has picked out some issues, and he's worked hard at them. But he has not developed a reputation as a deep, inside legislative type."

In seeking the presidency, Biden is not so much selling his record or a blueprint for the future as he is offering himself -- his personality, his passionate appeal, his Irish Catholic values, his New Generation theme, his confidence that he can rouse the country toward a sense of social justice.

Given his penchant for soul-searching monologues and rambling speeches, it could fairly be said that Biden is trying to talk his way into the White House.

Biden talks at length about the recurring suggestions that he flits from issue to issue.

"I think the perception on this score is inaccurate," he said. "My style is very different. I can name on one hand in 14 years the number of press conferences I've called in Washington.

"I think my legislative record is superior to the other candidates," Biden said. "What the hell have they ever passed? I'm not specialized. But I know more about drugs, arms control, foreign policy generally, the courts -- or as much, I should say -- as anybody in the United States Senate.

"There's been no self-promotion on this stuff, and it's a mistake," he said. "My focus has been Delaware."

As for his lack of a detailed agenda, Biden said: "I don't think presidents get elected on specifics. I think presidents get elected on broader notions of what their vision for America is."

Joseph Robinette Biden Jr. was born in Scranton, Pa., in 1942, the son of an auto dealership manager. When he was a high school student in Wilmington, Del., Biden joined sit-ins at the segregated Towne Theater. By the time he was a freshman at the University of Delaware, "he talked about being senator and president," recalled Donald R. Brunner, his college roommate.

Biden dropped off the football team after one year because his father was dissatisfied with his B-minus grades. "Joe was the kind of guy who could read someone else's notes and do better on the exam than the guy who made the notes," said Brunner, now a Wilmington banking executive.

Biden also struggled at Syracuse Law School, where he spent much of his time skiing and "almost flunked out." He said that he found law school "the biggest bore in the world" and squeaked through by "doing all-nighters all the time."

After opening a Wilmington law practice, Biden's political career had progressed as far as a two-year stint on the New Castle (Del.) County Council. But in 1972, when no big-name Democrat dared challenge veteran Sen. J. Caleb Boggs (R-Del.), Biden saw his opening and jumped into the race. With help from pollster Patrick Caddell and consultant John Marttila, who continue to advise him, Biden upset Boggs by 3,000 votes. He was 29. Personal Tragedy After His Election

A month later, Biden's wife and baby daughter were killed and his two sons seriously injured in an auto accident. He entered the Senate reluctantly and began a daily, four-hour round-trip commute from Wilmington to be with his sons. Biden's life remained disorganized until he met his second wife, Jill, whom he married in 1977. He still takes the train home every night.

Biden has made good use of his committee assignments -- Judiciary, where he is the ranking Democrat, and Foreign Relations -- but has showed signs of impatience. He repeatedly expressed frustration during his eight years on the Budget Committee and resigned from it last year.

"It became clear to me when Reagan became president that it no longer had much relevance . . . . The last five years the committee's made no difference," Biden said. Many disagree, pointing out that the Budget Committee has often helped redirect federal spending in ways the White House has opposed.

In any case, Biden concedes he made a frivolous impression in the Budget Committee by acceding to the Democratic leadership's plea that he "just show up. So I'd walk in and, bang, I'd get up and leave."

On Foreign Relations, Biden won rave reviews for his tenacious questioning of witnesses on the ill-fated SALT II treaty, which he supported. Alton Frye of the Council on Foreign Relations described Biden, a centrist on defense issues, as "the liveliest mind at work on the panel."

But after mastering the intricate details of arms control in 1979, Biden remained virtually silent on the subject until this June, when he cosponsored legislation to block President Reagan's attempt to abandon the treaty limits. Biden makes no apology for dropping out of the debate.

"You could be up to your ears in alligators on arms control in 1982, '83, '84 and '85, and it's a waste of your time," he said. "There's nothing you're going to affect. The reason why I'm back involved in it now? Turning point. Because this is the only time to be involved. You can affect something now. "He Skipped the Diplomatic Niceties

Biden waded into international waters in 1984 when he and Cohen, on a private visit to Moscow, carried a message from Reagan about a new approach to arms control. Cohen recalled Biden telling Georgi Arbatov, head of the Soviets' USA-Canada Institute, " 'All right, Georgi, cut the crap. Your economy is in a desperate situation, your philosophy is dead, no one's buying that song and dance anymore.' He brushed aside with one sweep of the hand all of the formal diplomatic niceties."

This kind of candor has gotten Biden in trouble, and Cohen said that "he opens himself up to having those words turned against him. He's probably more cautious now. Part of that may be ambition."

Biden has displayed a knack for building coalitions, particularly on the Judiciary Committee, where he is sandwiched between more liberal Democrats, Edward M. Kennedy (Mass.) and Howard M. Metzenbaum (Ohio), and conservative Republicans from Hatch to Chairman Strom Thurmond (S.C.). Biden has cultivated a warm relationship with Thurmond, ignoring advice that he be more partisan.

This proved invaluable when Biden helped secure passage of the 1984 Comprehensive Crime Control Act. He got the Democrats to agree to strengthen forfeiture laws and allow judges to hold more defendants without bail; he persuaded the Republicans to drop such controversial provisions as a federal death penalty, and he made sure Thurmond got most of the credit. Civil liberties groups said the measure could have been far worse without Biden.

"His goal in life is not to have a string of bills with pens on his wall," Hatch said of Biden, "but he has had an imprint on almost every piece of major legislation that's come through the Judiciary Committee."

Liberal and civil rights groups grew impatient with Biden during the first five years of Reagan's tenure for refusing to take on the president's conservative judicial nominees. Although a longtime supporter of civil rights, Biden did not emerge as a leading advocate until recent years. And his relations with the civil rights movement have occasionally been strained since 1975, when Wilmington was engulfed in a massive school busing case and Biden sponsored the first Senate-passed amendment to restrict busing.

Biden likes to spotlight these deviations from the liberal line, and he can be brusque with outside groups. "He lumps all civil rights and public interest lobbyists together as one special interest, and he wants so badly to be seen as not beholden to those interests," said a liberal activist who finds Biden "abrasive."

Last summer, however, Biden was hailed as a civil rights hero when he led a skillful effort to block a promotion for Justice Department civil rights chief William Bradford Reynolds. Later, under pressure from Kennedy and Metzenbaum, Biden began to battle some of Reagan's judicial nominees as unqualified and/or too extreme.

He has since won national attention by defeating Sessions, an Alabama prosecutor, and twice coming within a single vote of defeating Manion, an Indiana lawyer who was confirmed last week after an all-out campaign by the White House.

"Joe reached a point in his career where he faced a political ultimatum: You either lead the charge or get run over by it," a Republican Senate aide said. "He became more hard-nosed, more intransigent."

Biden insists he is not comfortable attacking presidential nominees -- "I was a defense lawyer, not a prosecutor," he said -- although he first reaped a publicity bonanza last year for his long-winded attack on Edwin Meese III's nomination as attorney general. That became the model for what reporters call Biden's "Hamlet" speech.

Here is Biden last fall, telling Charles J. Cooper of his doubts about Cooper's nomination to be an assistant attorney general: "Do you understand what bothers me most about what is happening at the Department of Justice? . . . See, that is what guys like me think of you all . . . . I get the feeling that you all are down there with your glasses on and your green eyeshades, late at night, sitting there saying, 'Now how can we screw up the Constitution?'

" . . . It is your mind-set that bothers me, and I am not sure -- I must admit to you -- I am not sure that my strong disagreement with your views, what appear to be your views on civil rights, I am not sure that is not coloring my view of your actions . . . . I admit ahead of time, I would sure like to be able to vote against you because I have such strong disagreements with you on your ideological disposition . . . . "

Said one Senate Democratic staffer who respects Biden: "He talks too much. Sometimes he can be remarkably articulate, and sometimes you roll your eyes to the back of your head." 'My Swing Is Natural for Me'

Asked about such criticism -- that he is too self-centered, too wrapped up in his own eloquence and image -- Biden is undeterred. In an anecdote-filled response that lasted almost five minutes, he compared politics to baseball and insisted that, "My swing is natural for me . . . . I can't measure out passion in minutes. If I talk too long, well, I have to talk and say what I think."

While Biden the senator insists that he is not opposing Reagan nominees on ideological grounds, Biden the national candidate told the NAACP this month that he is "engaged in all-out warfare with the right wing in this country" and that "the right-wing ideologues of this administration are . . . packing the courts with ideological robots programmed to respond to the whip of the radical right." He did not mention that he voted for all but five of those judges.

Biden's soliloquies have also raised eyebrows in the abortion debate. Asked where Biden stands on abortion, Jeannie Rosoff, president of the prochoice Alan Guttmacher Institute, said: "I think he always stands wringing his hands. He sort of anguishes all the time. I've never been able to figure out if he's personally anguished or politically anguished. He talks about what great pain it gives him, how difficult it is, how he's torn and so on. I'm a little suspicious of so much anguish."

At the same time, some antiabortion groups have campaigned against Biden. "He favors unrestricted legal abortions, which he didn't used to," said Douglas Johnson of the National Right to Life Committee. "At one time we would have regarded him as leaning in our direction."

Biden said he supports the right to abortion but votes against federal funding to pay for it. "It's the only consistent position intellectually, which is that if you say government should be out, then government should be out," he said.

In Delaware, where Biden was reelected in 1984 with 60 percent of the vote, people seem tickled at the notion that a local boy is viewed as presidential material.

"The man wears well," said state Democratic Party Chairman Sam Shipley. "You can't be a statewide figure in a state as small and personal as this and not wear well. People see him get on the train every night and come home. "In Delaware, He's Good Old Joe

Despite an initial reputation as a "pop-off artist," a Wilmington political observer said, the senator has become "phenomenally popular in Delaware. He's never been thought of here as a heavy thinker. They think of him as Good Old Joe."

Good Old Joe almost made a run for the White House in 1983, when his friend Caddell brought him polling data showing a political vacuum that a younger candidate could exploit. Biden decided not to make the race; Caddell took his analysis to Sen. Gary Hart (D-Colo.), who almost won the Democratic nomination.

Biden would be "a much more passionate and emotional candidate" than the more analytical Hart, said Marttila, who is plotting Biden's White House strategy. "A lot of his potential strength as a candidate is he's a powerful speaker who can really move audiences. He can reawaken the conscience of the country."

That is the Biden boosters' view of his candidacy: a charismatic young senator, his life touched by tragedy, who likes to invoke the legacies of John and Robert Kennedy and the Rev. Martin Luther King Jr. But they also complain that Biden's oratorical skills and overtaxed schedule have obscured his legislative talents.

"Even on the days when he hasn't read every word in the notebooks we've prepared for him, he is terrific about picking up the gist of something," said Cindy Lebow, the Judiciary Committee's Democratic staff director. "He likes to view himself as a trial lawyer extraordinaire. He's quick to analyze things, quick to sense where the jury is turning."

Whether the American jury will render a favorable verdict on Biden, his strategists say, will ultimately depend more on his ability to inspire than on the number of bills that bear his name.

But Biden thinks that he must first pass a basic credibility test with the national news media, one in which he displays himself to the public "warts and all. Now's the time for me to sort of take the shirt off and say, 'Go ahead and look.' It ain't gonna stay open all the time."