A fierce political confrontation that has reignited many of the passions that swirled around Ronald Reagan's first election as president in 1980 begins in earnest Tuesday when the Senate Judiciary Committee opens hearings on Reagan's nomination of Robert H. Bork to the Supreme Court.

Bork, a judge on the U.S. Court of Appeals here and a former Yale Law School professor known for his intellect, his outspokenness and his charm, has been the source of intensifying controversy since Reagan nominated him on July 1. Activists on the right and the left have launched unprecedented campaigns for and against Bork, including widespread public opinion polling, paid media campaigns and grass-roots organizing by dozens of organizations.

Both sides predict a close vote on the Senate floor later this year. The outcome could hinge on the ability -- and the willingness -- of Senate Democrats to sustain a filibuster against the nomination under enormous pressure from the White House.

Before then, the Senate is likely to engage in a bitter debate over the direction of social and political policy for the rest of this century.

To Bork's liberal opponents, his confirmation to succeed retired justice Lewis F. Powell Jr. would tilt the closely divided Supreme Court sharply to the right, threatening a reversal of key decisions on civil rights, civil liberties, free speech, abortion, prayer in school and more. Bork's critics charge that Reagan hopes Bork can impose the conservative "social agenda" he promised in 1980 but has largely failed to deliver.

"This is his last gasp, and they know and we know it," said Kate Michelman, a Bork opponent who is executive director of the National Abortion Rights Action League.

But to Bork's defenders, the assault on his qualifications is driven by partisan politics and the "narrow special interests" that Reagan has twice decisively defeated at the polls. It is the liberals, Bork's supporters say, who are making a desperate last stand because Bork does not meet their rigid ideological standards for the court.

"The liberals, if they lost the court, are done for," said Daniel L. Casey, executive director of the American Conservative Union. "They have to start from scratch and build a political consensus on issues that they can't build on -- pornography, abortion on demand, racial quotas, busing."

The debate over Bork will begin to reach a wide public when his confirmation hearings begin Tuesday at 10 a.m. in the giant Senate Caucus Room. Bork's performance in the spotlight is seen as critical to his confirmation; public attitudes may also be shaped by perceptions of the judge's critics and allies on the sharply divided Judiciary Committee, which consists of eight Democrats and six Republicans. Both sides will be trying to reinforce their version of the radically different portraits that have been drawn of the 60-year-old jurist since his nomination.

Is Bork, as his supporters maintain, a brilliant legal scholar, a "mainstream" conservative who believes deeply in "judicial restraint" and who would deal with each case fairly and with an open mind?

Or is he, as the critics charge, a closet "judicial activist" on the extreme right who would try to impose an exceptionally narrow view of the Constitution on many issues?

Despite the deep feelings generated by the nomination, both sides appear eager to make the hearings as noninflammatory as possible, each side believing such an approach is in its best interests. For example, Senate sources said Sen. Edward M. Kennedy (D-Mass.) has been urged not to repeat his initial outburst against the nomination, when he said Bork's confirmation would bring a return of "back-alley abortions" and "segregated lunch counters," or to subject Bork to the kind of hostile questioning he gave Chief Justice William H. Rehnquist at his confirmation hearing last year.

Similarly, pro-Bork strategists say the nominee has been cautioned not to flaunt his intellectual abilities or to appear flip or condescending in answering questions.

Both sides have spent weeks preparing for the hearings.

"It's like a championship fight," said veteran lobbyist Tom C. Korologos, who is helping Bork. His preparations have included mock hearings at which Bork fielded questions from administration officials and outside supporters, including Lloyd Cutler, White House counsel under President Jimmy Carter.

Unlike previous confirmation hearings, in which senators gingerly pressed nominees for their judicial views about specific issues, the hearings this time are expected to produce detailed questions and answers. And according to Korologos, Bork will abandon the traditional Supreme Court nominee's reluctance to answer substantive questions.

"He's going to be a lot more candid than Rehnquist and {Justice Antonin} Scalia were," Korologos said. "He'll answer all their questions up to 'How would you vote on?' " specific cases, he said.

Korologos predicted committee Democrats "are going to let him go easier than we think. They don't want to Ollie Northize him."

Bork supporters insist their best weapon is Bork, who both sides acknowledge is engaging, witty and bright.

Republicans hope that committee liberals try to debate him. "If they were smart," said Daniel J. Popeo, general counsel of the conservative Washington Legal Foundation, "they wouldn't let him testify at all. Bob Bork is going to clean the floors with Joe Biden," referring to Sen. Joseph R. Biden Jr. (D-Del.), chairman of the Senate Judiciary Committee.

Conservatives worry, however, that Bork, confronted with tedious or repetitive questions, may appear high-handed or condescending, traits that would not sit well with the viewing public. But Korologos plays down the theatrical.

"Our goal is three undecided votes on the Judiciary Committee and 50 {to} 60 if there is a filibuster in the Senate," Korologos said. "He's not running for mayor or senator or Miss America."

Biden, whose presidential ambitions may well hinge on his performance, has been preparing for weeks for the hearings. He said last week he has read "every single word" that Bork has written.

Biden disassociates himself from the more strident liberals on the committee who have accused Bork of being a radical ideologue. "I have not characterized him that way in any speech that I have made, any comment I have made or anything else," Biden said last week at a luncheon meeting with reporters and editors of The Washington Post.

Biden disputed Bork's view that the Constitution guarantees no right to privacy, but avoided attacking him for his position on any particular issue. Bork is "intellectually very narrow" and wrongly interprets the Constitution and Bill of Rights, Biden argued. "I think the Constitution is an ennobling document. I think it is meant to lift up the country. I don't believe it is as pedestrian as {Bork does}."

On the eve of the hearings, there have been no indications that investigations into Bork's background have produced embarrassing or compromising material. Senate sources said the most serious setback for Bork so far was the American Bar Association's divided vote on the nomination. Four of the 15-member ABA judicial evaluation committee found him "not qualified." A fifth member voted "not opposed." The committee has unanimously endorsed all other recent Supreme Court nominees.

Bork is certain to be questioned closely about his role in the "Saturday Night Massacre," when as solicitor general under President Richard M. Nixon he fired Watergate special prosecutor Archibald Cox after Attorney General Elliot L. Richardson and Deputy Attorney General William D. Ruckelshaus resigned rather than do so. But strategists on both sides say they do not expect Bork's Watergate role to be a pivotal issue.

Nor does either side expect the hearings to produce the kind of trial-like atmosphere that Rehnquist faced when he was grilled about allegations that he attempted to intimidate minority voters in Arizona in the early 1960s. "It will be a very high-minded policy fight," said a Democratic Judiciary Committee aide.

Bork's opponents hope to establish during the hearings that Bork, if confirmed, would become the critical fifth conservative vote on the court, forming a majority that is eager to "turn back the clock on civil rights and civil liberties," said Ralph G. Neas, executive director of the Leadership Conference on Civil Rights.

The critics emphasize the importance of Bork as a successor to Powell, often the decisive vote in major decisions. By the end of the hearings, Neas said, it should be clear that, unlike Powell, "Robert Bork is neither a moderate nor a conservative. Robert Bork is a judicial activist with a right-wing agenda."

"The most effective argument against Bork is that he is an extremist on a variety of issues," Democratic pollster Harrison Hickman wrote in a memorandum to the National Abortion Rights Action League. Hickman based his conclusions on "focus group" interviews with voters in Pennsylvania and Alabama, an example of the intense preparation both sides have made for the battle.

To establish that Bork falls outside what Hickman called "the general values and consensus views" that voters look for in Supreme Court nominees, Bork's opponents plan to turn his words against him. In an extraordinarily prolific career as a lawyer, law school professor and, since 1982, appeals court judge, Bork has left "a 25-year paper trail that can't be changed, forgotten or shredded," Neas said.

Bork has criticized Supreme Court decisions that legalized abortion, struck down racially restrictive covenants, established the "one man, one vote" principle in elections and barred the forced sterilization of certain criminals. In the early 1960s, he also criticized passage of civil rights legislation opening public accommodations to blacks, a position he recanted when he was confirmed as solicitor general in 1973.

Senate Democratic sources say Bork has backed off several other controversial positions in private meetings with senators since his nomination. Biden said there is "a real possibility" that Bork will tell the committee, " 'I was just being provocative and disregard everything I've written and said for 25 years.' "

Korologos, however, said, "You aren't going to see him change his philosophy 48 hours before the hearings; the Senate can spot a phony in a minute."

For many Bork supporters, his nomination offers the possibility that the "Reagan revolution" could blossom on the Supreme Court long after the president has left office.

"Especially on the right, they have been so demoralized," said Eddie Mahe Jr., a Republican political consultant. "Much of what they thought they were going to accomplish hasn't even been on the table. But if they win {on Bork}, they have themselves another hero."

The administration initially insisted that Bork should be judged only on the basis of his personal and professional qualifications, and that political ideology should not be a consideration. But it appears to have lost on this point; a new Justice Department statement supporting Bork released today implicitly acknowledges that Bork's positions on constitutional issues are part of the confirmation debate.

The Judiciary Committee appears divided along ideological lines. Five of the committee's Democrats -- Biden, Kennedy, Howard M. Metzenbaum (Ohio), Patrick J. Leahy (Vt.) and Paul Simon (Ill.) -- are considered virtually certain to oppose the nomination. Bork can almost certainly count on the support of five committee Republicans -- Orrin Hatch (Utah), Strom Thurmond (S.C.), Alan K. Simpson (Wyo.), Charles E. Grassley (Iowa) and Gordon J. Humphrey (N.H.).

Three committee members considered genuinely undecided are the objects of the most lobbying -- Arlen Specter (R-Pa.), Dennis DeConcini (D-Ariz.) and Howell T. Heflin (D-Ala.).

Strategists on both sides say they consider Specter the most likely of the three to oppose Bork, and DeConcini the most likely to support him. DeConcini told The Arizona Republic last week that he is concerned that Bork is "stuck ideologically" and an advocate of his own brand of "judicial activism," but his press secretary said later it would be "incorrect" to conclude that DeConcini is leaning for or against the nomination.

Both sides also agree that, of the three, Heflin is the most important. A southern conservative and former chief justice of the Alabama state supreme court who almost never tips his hand on an important issue, Heflin is seen by Bork's opponents as a vital link to other southern Democrats, the most important bloc of votes on the Senate floor.

Heflin's opposition to the nomination, a pro-Bork strategist acknowledged, could be crippling if not fatal.

Although the Judiciary Committee will not decide the issue, its vote could have a major impact on the Senate floor. With the committee apparently so evenly divided, there have been widespread predictions it will opt to send the nomination to the floor without a recommendation.

Bork's opponents are reluctant to discuss the possibility of a filibuster, insisting that they can put together the 51 votes necessary to defeat the nomination outright. But if it appears they could fall short of that goal, an aide to Biden said, they will face "a very difficult tactical decision" on resorting to a filibuster, a tactic that many Democrats fear could backfire on them in next year's elections.

Southern Democrats are one of two groups of senators that appear vital to the outcome of the confirmation fight. The other consists of moderate Republicans like Sen. Lowell P. Weicker Jr. (Conn.), one of only two Republicans who opposed Rehnquist, and Sen. Bob Packwood (Ore.), a strong abortion-rights advocate who has said he will base his decision solely on Bork's views on the abortion issue.

Bork's opponents will need to win at least a handful of the moderate GOP votes to defeat the nomination.

The southern Democratic bloc includes five freshmen who owe their elections last year largely to the overwhelming support of black voters. These five -- John Breaux (La.), Wyche Fowler Jr. (Ga.), Bob Graham (Fla.), Terry Sanford (N.C.) and Richard C. Shelby (Ala.) -- could hold the balance of power in the final showdown on the Senate floor.