John Kennedy, the handsome, sexy, liberal scion of a newly rich Eastern Irish Catholic family, simply seduced the nation. So says author Garry Wills, 47, a porky, vigorously independent Midwestern Irish Catholic, who is the latest writer to tromp through JFK's 35-month Presidency and find Camelot a ghost town. But in The Kennedy Imprisonment: A Meditation on Power (Atlantic-Little, Brown, $14.95), Wills has a novel thesis. The Kennedys' vigor has faded, he writes, because their patriarch passed on a virulent social disease: He had "no ideology but achievement."

In the past Wills' fascination with "how leaders sum up the national hopes and illusions" has led to such notable works as 1970's Nixon Agonistes and Inventing America, his 1978 reappraisal of Thomas Jefferson. His new book is a family portrait—and a dark one. Joseph P. Kennedy Sr. set his clan's buccaneering style with his financial wheeling and dealing, Wills says, and with his philandering, which included a dalliance with Gloria Swan-son as well as approaches to his sons' girlfriends. Wills relates an episode in the 1940s when John, then a Navy intelligence officer in Washington, was so involved with a Nazi favorite named Inga Arvad that he attracted the attention of the FBI. Though Old Joe disapproved, and had to ask Navy Under Secretary James Forrestal to keep Jack from being bounced from the service, he also had a yen for Inga. Her son recalls being told that Kennedy Sr. would try "to hop in the sack with her" whenever Jack left them alone.

In Wills' view, JFK translated Joe's swashbuckling style into a "cult of courage" that led on the one hand to the 1961 Bay of Pigs debacle and on the other to his own relentless pursuit of women. One of them, Judith Campbell Exner, a sometime Frank Sinatra mistress, tattled that Jack was forever pumping her for Hollywood gossip—"Who's Frank seeing now?" or "I heard Frank is seeing so-and-so, and isn't she married?" Concludes Wills: "Joseph Kennedy, a man of strong will and low tastes, passed on both traits to his son."

Wills believes the excesses of the elder Kennedy and JFK hobbled the younger brothers. The "puritan" Robert was forced to become a scrapper and came to "resemble his predator father more than any other son." But it was Edward, "the least manipulative of the Kennedys," who suffered the most, Wills argues. Joe and JFK were "far more single-minded philanderers" than Teddy, but he is stuck with the bad-boy image. Whatever the circumstances of Mary Joe Kopechne's 1969 drowning, Wills asserts, Kennedy had gone to Chappaquiddick merely to do family duty by appearing at a reunion of aides to his slain brother Bob. Wills also suggests that the collapse of Ted's marriage was scarcely all his fault. He cites the searing experience of a chum who was visiting at Hyannis Port and was taken by Kennedy to see wife Joan slumped in the back seat of a car: " 'She was a rag mop,' the friend observes...'the result of a two-or three-day bender. I think Kennedy just wanted me to see what he was up against.' "

Teddy's tragedy, Wills concludes, is that he has managed to outlast his brothers "without ever catching up to one of them." In the 1980 campaign he was "forced, every day, to demonstrate he was not as good as his brothers. [But] he could not make the counterclaim" that his brothers weren't all that good, "that Camelot had been a fabric of political unreality."

The son of an appliance salesman, Garry spent seven years studying classics as a Jesuit seminarian at St. Louis University. A critique of TIME style that Wills sent to William F. Buckley Jr. got him a job offer from the National Review in 1957, but he chose just to do free-lance pieces while earning a Ph.D. in classics at Yale. On a flight to New York to see Buckley, Wills met a Nietzsche-reading stewardess, Natalie Cavallo. They wed in 1959. Though Buckley was an usher, he scourged Wills as "a turncoat" after he came out against the Vietnam War in 1969.

At Northwestern, where he is a popular lecturer, Wills delights in being tough to pigeonhole. Though conservative on some issues, he advocates unilateral nuclear disarmament. "What's more conservative than wanting to conserve the world?" he asks, invoking the interests of his kids' generation. (John, 22, is a history major at Holy Cross, Garry, 20, a classics student at Yale, and Lydia, 18, an Eli freshman.)

Wills himself did little interviewing for his book. "I don't hang around politicians—I hang around libraries," he explains. Indeed, most of his facts were gleaned from others' works. But the overall interpretation of Camelot is pure Wills. Does he expect to hear from any old New Frontiersmen? Not really, he says. "People hate you most when you pity them."