REPUBLICAN leaders may have breathed a sigh of relief at David Duke's recent defeat in the Louisiana gubernatorial race, but they quickly reverted to form by again engaging in the divisive, stigma-ridden attack politics developed by their own candidates and then adopted by Mr. Duke.Election returns in Louisiana were still being counted when Vice President Dan Quayle and White House spokesman Marlin Fitzwater flagrantly played to nativist prejudices by hammering away at New York Gov. Mario Cuomo's Italian (and obviously Roman Catholic) heritage: "Mario, Mario, Mario, Mario, Mario. That's his name - he'd better get used to it," Mr. Fitzwater taunted. Sen. Phil Gramm (R) of Texas, never one to avoid the alarmist tactic, has similarly observed, "We don't have many Marios down [in Texas]." Get it? Most of us thought such prejudices went out with 19th-century Thomas Nast drawings of Catholics as Papist conspirators against the Constitution. Not in the GOP, apparently. Just two weeks later, the day before the president was to sign the historic civil-rights bill, Bush strategists were planning to gut the rules governing affirmative action. Embarrassed by the public disclosure, George Bush announced his support for the affirmative-action programs. Over the past decade and a half, Republican strategists have perfected the use of invective, smear, and innuendo that persuaded nearly 40 percent of Louisianans that a former Nazi and Klansman ought to be governor. Just as manufacturers are liable for damage caused by products they place in the stream of commerce, political leaders must be held accountable for the themes they employ to manipulate public fears. For a quarter century, Republicans have employed divisive images in their campaign rhetoric to capitalize on middle-class anxieties and to entice the right-wing constituency. Since the Nixon-Agnew drumbeat of "law and order" in the 1968 campaign (adapted from the racist appeal of George Wallace), Republicans have shamelessly sought to associate Democrats (and especially liberal Democrats) with issues such as crime, preferential hiring, civil- and voting-rights laws, and welfare to inflame racial prejudices. In unguarded moments, Republican leaders acknowledge their comfort with the politics of alienation and innuendo. Mr. Quayle, in an uncharacteristic moment of insight, recently observed that "the problem [with Duke] is the messenger," not the message - a sobering distinction for those who had hoped that the administration's objections rested on more than Duke's organizational affiliations. Indeed, most of Duke's recent campaign rhetoric squares quite nicely with that of Republican conservatives. "The welfare system is a source of our crime, a great deal of our violence, a great deal of the problems in our educational system," Duke declared. Duke's images of welfare mothers breeding to qualify for larger checks conforms nicely to Ronald Reagan's highly effective image of a welfare mother tooling around Sacramento in her Cadillac. In mid-October, Duke lambasted Buddy Roemer, Louisiana's outgoing governor, for commuting the sentence of convicted murderer Cleveland Harris, this election's incarnation of Willie Horton, who was then paroled and who subsequently committed another murder. When Duke assailed Mr. Roemer for being "too liberal," he sounded an awful lot like Bush pummeling Michael Dukakis with buzz words like "the American Civil Liberties Union,liberals," and "Massachusetts." Indeed, Duke took Bush at his word that the opposition to all of these evils is "mainstream Republican." The chilling similarities in political themes and campaign styles do not make Republicans neo-Nazis or Duke a mainstream Republican. But Duke's speeches illustrate how, when demagoguery becomes the common currency of public discourse, it has an ugly and uncontrollable way of steering politics and voters. Contrary to the vice president's sage insights, the issue isn't just Duke's memberships or even his messages of divisiveness and prejudice. The truly frightening concern is the easy acceptance of Duke's underlying assumptions by millions of voters and by one of our nation's major political parties. If the discomfort Duke generated is allowed to fade as quickly as recent rhetoric suggests, we will have lost an important opportunity to find real solutions, not scapegoats, for the complicated problems that confront America.