LAFAYETTE, LA., OCT. 3 -- It is safe to say that Louisianans know the real David Duke by now, but will that knowledge make much difference when they decide whether to send him to the United States Senate? Two days before the final and most unusual contest of the 1990 primary election season, that is still the essential question -- not only for his main opponent, Democratic Sen. J. Bennett Johnston, but for a Republican Party that sees Duke as its own worst nightmare and for a nation that is struggling anew with issues of race.

Week after week for the past six months, Duke has been exposed in the press and in television commercials produced by his adversaries not only as a race-baiting zealot but as a fabricator of his past and present. The list of embarrassing revelations is long and diverse.

Here is a candidate who did not pay federal income taxes from 1984 to 1987. Here is a 40-year-old man who underwent facial reconstruction and chemical peels to enhance his TV image. Here is a self-described patriot who claimed that he served behind enemy lines in Laos during the Vietnam era, but who never served in the military, avoided the draft, and spent only a few weeks teaching English to Laotians while visiting his father, a former foreign service officer.

And more: The same Duke who is appealing for the veterans vote once said the United States had no business getting into World War II, hired a longtime member of the American Nazi Party to run his 1988 presidential campaign, and as recently as last year sold pro-Hitler books from his legislative office. The same Duke whose stump speech mocks the sexual habits of welfare recipients once wrote a primer urging women to perform oral sex to keep their mates.

From the time Duke was elected as a Republican to the state House nearly two years ago, most voters here had at least a vague understanding that he was a former leader in the Knights of the Ku Klux Klan. Last week the image was burned into the state's consciousness in an unforgettable way: Johnston's media advisers saturated the airwaves with TV spots showing old film of Duke addressing his hooded and robed followers at a cross-burning ritual.

"I think Duke's been damaged by all this," said Lance Hill, director of the Louisiana Coalition against Racism and Naziism. "If this isn't sufficient information on which to reject him, I don't know what would be."

There is conflicting evidence as to how much Duke has been damaged going into Saturday's primary voting. Johnston's camp claims that after the Duke-as-a-Klansman commercials ran, Johnston, seeking his fourth term, advanced 4 points in the polls and Duke lost 4 points -- enough of a swing, they claim, to make it possible for Johnston to win the 50 percent or more he needs Saturday to avoid a November runoff. Independent pollsters are less certain that Johnston has clinched victory in the three-way open primary against Duke and the official Republican candidate, the all-but-forgotten state Sen. Ben Bagert of New Orleans.

"My sense is that the aggregate has stayed the same: Duke has held his ground," said Susan Howell, director of the Survey Research Center at the University of New Orleans. Her last poll showed Johnston with 46 percent, Duke 21 percent and Bagert 8 percent, with 20 percent undecided and 5 percent refusing to answer.

Although Duke's 21 percent marked a slight decline from previous polls, Howell said it was inconsequential and would be made up for on Election Day by two factors. First, she said, there is a "hidden Duke vote" among those listed as undecided or refusing to answer -- people who support Duke but are reluctant to express that support publicly. That group, Howell said, is largely within the professional class. And then there is what Howell called "the extraordinary intensity" of Duke support among working-class whites. Howell said she would not be surprised if 70 percent of Duke's supporters vote on Saturday, whereas Johnston will need a concerted effort to get out 45 percent of his vote.

The intensity of Duke's support was apparent Tuesday night in a campaign rally at the Yellow Rose country dance hall on the edge of Lafayette, where 250 people in the heart of Cajun country belted out "Dixie," recited the Pledge of Allegiance, sang the National Anthem, then gave Duke a rousing reception for his standard attack on welfare recipients, affirmative action, integration, taxes and the corporate and government establishments.

Surveys indicate that Duke is most popular among poor and uneducated whites, but the crowd at the Yellow Rose was more diverse. Walking through the smoke-filled hall, one encountered in succession a slice of working Louisiana: A federal inspector for Section 8 public housing, an architect, a truck stop owner, an oilfield engineer, a real estate broker, two caterers, an exterminator, a firefighter, five college students and an oil and gas production supervisor.

Many of the men were veterans of World War II, Korea or Vietnam. Although Duke has been disparaged by several veterans organizations as a draft dodger with Nazi sympathies, the vets at the Yellow Rose were untroubled by his past. "We'd all be better off if we stopped worrying what Mr. Duke's background is all about and started worrying about having to teach our grandchildren Japanese," said architect Jaco Leblanc, 62, of Breaux Bridge, a World War II veteran who was on a ship that sank in the Pacific with the loss of 500 men.

"He's going to get 80 percent of the veteran rank-and-file," said Korean War veteran Bill Castellini, vice chairman of the St. Landry Parish Republican committee. "It doesn't bother me what they say about him -- at least he was over there in Southeast Asia."

Johnston, who started the campaign underestimating Duke, has now spent more than $2 million trying to avoid a runoff with him. With his campaign appearances poorly attended and lackluster, the Shreveport Democrat has relied increasingly on a television campaign directed at Duke's background.

The key to victory for Johnston, as it was for Democratic Sen. John Breaux in 1986, will be the turnout among blacks, who make up about 26 percent of the total vote. Polls show Johnston with 69 percent of the black vote, 3 percent for Duke and 5 percent for Bagert. Among whites, Duke is within 10 points of Johnston -- 38 percent to 28 percent.

Johnston's supporters were buoyed by high absentee balloting last week in New Orleans, where black voters waited in line for two and three hours to cast their ballots, lured not only by the Senate race but by a heated congressional contest involving several black candidates.

Bagert, the endorsed GOP candidate, is hoping for a miracle. He is optimistic that voters disenchanted with Johnston for his centrist, Democratic voting record and opposition to Supreme Court nominee Robert H. Bork but embarrassed by Duke will vote for him in the end. But a miracle looks unlikely. Bagert's two top campaign officials resigned earlier this month, complaining that the candidate was inept, preferring to stay at home and play the guitar rather than campaign through northeastern Louisiana.

"We could have won this election with the right candidate," said state GOP Chairman Billy Nungesser, who attacked Duke as a "draft-dodger" last week. "The vote is just sitting there and ain't happy where it's at."