Lee Atwater, former chairman of the Republican National Committee, manager of George Bush's 1988 presidential campaign and a principal architect of the GOP's domination of presidential politics during the last decade, died yesterday morning at George Washington University Hospital of a brain tumor. He was 40 years old.

Trained in the politics of the Deep South by Sen. Strom Thurmond (R-S.C.), Atwater embodied in strategy and spirit the ascendant Republican Party that rose from the ashes of the landslide GOP defeat in 1964. He relished operating on the edges of propriety and to members of his party he was a genius in defining and exploiting the weaknesses of his opponents.

To his critics, however, he was a symbol of the dark side of American politics. To them, his success in elevating a black murderer-rapist named Willie Horton into a national figure used to crush the presidential bid of Michael S. Dukakis was a crude appeal to racism and the epitome of the negative campaign.

In recent months, as his illness worsened, Atwater expressed regret for some of his past activities. "In 1988, fighting Dukakis," he wrote in Life Magazine, "I said that I 'would strip the bark off the little bastard' and 'make Willie Horton his running mate.' I am sorry for both statements: the first for its naked cruelty, the second because it makes me sound like a racist, which I am not."

Atwater also expressed second thoughts about the 1980s, the decade of his greatest triumphs. "The '80s were about acquiring -- acquiring wealth, power, prestige," he wrote. "I know, I acquired more wealth, power and prestige than most. But you can acquire all you want and still feel empty. . . . It took a deadly illness to put me eye to eye with that truth, but it is a truth that the country, caught up in its ruthless ambitions and moral decay, can learn on my dime."

President Bush said yesterday that Atwater had "practiced the art of politics with zeal and vigor" and that he and his wife, Barbara, were "heartsick" over Atwater's death. A spokesman said Bush plans to attend a memorial service for Atwater Thursday at Washington Cathederal.

Before he became ill last March, Atwater was the de facto leader of a generation of Republican strategists who cut their teeth in the divisive politics of the 1970s and came of political age with the Reagan revolution in the 1980s. Exceptionally candid, colorful, inquisitive and the master of his own charm, Atwater was considered by many to stand head and shoulders above the pack of Washington political operatives.

Atwater's specialty was the conversion to the GOP of once-loyal Democratic voters through the political use of such social issues as crime, gay rights, school prayer and abortion.

He thrived on the development of such so-called "wedge" issues to separate voters from their historic allegiance to the Democratic Party and to build a conservative alliance between white populists and the country club elite.

Atwater put these talents into practice in the 1988 presidential campaign against Dukakis. With the Massachusetts governor riding high in the polls, Atwater late in the primary season ordered the research staff of the Bush campaign to produce a short list of Dukakis's liabilities.

The staff returned with the now-famous case of Horton and his escape from a Massachusetts prison furlough program, with Dukakis's veto of legislation requiring public school teachers to lead students in the Pledge of Allegiance, with the governor's membership in the American Civil Liberties Union and with his opposition to the death penalty.

Armed with these issues, Atwater never doubted that Dukakis could be beaten -- even when he had a 17-point advantage in the polls -- and Bush's landslide victory that fall proved him right.

Immensely entertaining, talented and hard-working, Atwater began his political career by running state legislative and city council campaigns in South Carolina in the early 1970s.

He directed Thurmond's 1978 reelection bid, served as southern coordinator for the 1980 Reagan-Bush campaign, then as deputy manager of the 1984 Reagan-Bush campaign and as manager of the 1988 Bush-Quayle campaign. In 1989, Bush chose Atwater to head the RNC, and in January, when former agriculture secretary Clayton Yeutter succeeded him, RNC members voted to give Atwater the ceremonial post of general chairman.

Throughout his career, the focus of Atwater's attention remained on trends within the electorate, especially on voters trapped not at the bottom but on the lower and middle rungs of the socioeconomic ladder -- the swing electorate critical to the outcome of campaigns.

He also fancied himself an expert on his Baby Boom contemporaries, who, he said, are intensely sensitive to the "baloney" of politics. "If you want the Atwater theory of the 1988 campaign it is this," he said in a postelection interview: "Dukakis and his whole campaign were full of bull. They were a bunch of elitists sitting around who didn't have any idea of what was going on. . . . I can see through bull real quick. If there is one thing I really believe, that is that people are tired of bull. Bull has permeated our society at every level."

Atwater's basic strategies in campaigns evolved from his training as a Republican operative in South Carolina where, until recently, residual Democratic loyalty remained strong among white, middle-class and working-class voters.

"Republicans in the South could not win elections simply by talking about various issues," he said in an interview. "You had to make the case that the other guy, the other candidate, is a bad guy. . . . You simply could not get out in a universe where 60 percent of the people were Democrats and 28 percent Republican and win by talking about your issues."

Race was an integral part of this strategy, and exploiting racial fears and hostilities of white voters became a key factor in building a Republican majority in the South. Softening Atwater's image as a southern "redneck," however, was his well-publicized love of the blues guitar and his frequent performances with black singers and musicians.

When he became Republican Party chairman he vowed to reach out to black voters, but the deep animosity he had engendered among blacks was underscored when his nomination to the board of trustees of Howard University in 1989 caused such an outburst of campus protest that he was forced to withdraw.

Atwater, whose full name was Harvey Leroy Atwater, described himself accurately as a product of "the middle of the middle class." Born Feb. 27, 1951, in Atlanta, Atwater spent his earliest years in Charleston and Aiken, S.C., with his father, Harvey Dillard Atwater, an insurance claims adjuster, and his mother, the former Alma Page, known as Toddy. Both parents are still living.

Atwater had a younger brother, Joe, who died in a kitchen accident at the age of 3 when he tipped a pan of frying oil onto himself. Atwater, who was 5 and in the next room at the time, witnessed the death. "I can close my eyes and see it," he said in one interview, although he was never forthcoming about the event.

When Atwater was 10, his family settled in Columbia, S.C. At A.C. Flora High School, where he was by all accounts a poor student, Atwater put out a weekly mimeographed newspaper called "Big At's Comedy Ratings." The paper ranked classmates by how funny they could be, by the appeal of their personalities and by their popularity.

"With the comedy ratings, I was always cutting through the bull," he recalled.

Atwater's mother used all her ingenuity and connections to get him into Newberry College, a small, Methodist institution 37 miles northwest of Columbia. It was there that Atwater began to flourish intellectually. A year later, Atwater's mother used her influence again to get him a job as an intern in the office of Thurmond and, throughout the summer of 1971, Atwater drove with Thurmond around South Carolina, learning state politics from the master.

After graduating from Newberry in 1973, Atwater became executive director of the College Republicans in Washington, where he began his friendship with Bush, then RNC chairman. Bush won Atwater's loyalty for life by lending him his boat to impress a young woman working in Thurmond's office, Sally Dunbar. Atwater and Dunbar were married in 1978, and have three daughters, Sara Lee, Ashley Page and Sally Theodosia.

In early 1974, Atwater moved back to Columbia, where he managed his first statewide race, the failed attempt of former Vietnam War commander Gen. William C. Westmoreland to win the 1974 South Carolina GOP gubernatorial primary. The loss, Atwater said later, taught him to "never underestimate the power of the right" in a Republican primary. Westmoreland, a deeply conservative man, was battered and bruised by charges that he was a member of the "liberal" Trilateral Commission.

For the next two years, Atwater concentrated on small, local contests for legislative and councilmatic seats in a state where an almost nonexistent Republican Party was beginning its 16-year surge to ascendancy. In 1978, he renewed his efforts in statewide campaigns, picking a far stronger candidate than Westmoreland: Strom Thurmond. In was in this contest that Atwater began to earn his reputation as a tough -- and in the minds of some, dirty -- negative campaigner.

Charles Ravenel was the Democratic Senate nominee. According to Atwater, early in the campaign, he received a call from Roger Stone, another Republican consultant, saying that an alternative paper in New York, "something like The Village Thing," had quoted Ravenel at a plush, Park Avenue fund-raiser as saying "he was embarrassed to be from South Carolina and promised to be the third senator from New York." Said Atwater: "I felt like the guy standing on top of a 20-story building and seeing a $100 bill on the ground." In the anti-New York politics of South Carolina, Atwater pounded the airways with the Ravenal quote, which Ravenal denied having made.

In 1980, Atwater's instinct for the jugular sent a chill down the spines of even some friends and supporters when he described a South Carolina Democratic congressional candidate who had received electroshock treatment for depression while an adolescent as "hooked up to jumper cables."

After nearly 20 years of attack politics, Atwater indicated before his illness that he planned a significant shift in tactics. He told associates that as the vote-heavy Baby Boom generation entered its 30s and 40s, a growing concern with developing good relationships and more secure families would begin to replace acquisitiveness and the hunger for status and economic success.

These shifting attitudes, he argued in a 1989 interview, would force a change in national politics. In addition, Atwater said, he faced the danger of getting caught in a rut. "I'm not going to do this negative stuff again; I'm not going to leave a pattern," he said.

In one of the few interviews he agreed to after his cancer was diagnosed a year ago, Atwater told the Columbia State newspaper: "I can't imagine me getting back in a fighting mood. . . . It's going to be hard for me to be as tough on people. . . . Seventy percent of the things I was frantically pursuing didn't matter anyhow. . . . Forget power and money. I had no idea how wonderful people are. I wish I had known that before. What a way to have to find out."

He is survived by his wife and three daughters of Washington, his parents of Columbia, S.C., and a sister, Anne King of Charleston, S.C.