In “The Kennedy Half-Century,” political scientist Larry J. Sabato summarizes a recent poll that helps shed light on John F. Kennedy’s importance to Americans 50 years after his death. The survey, by Peter Hart and Geoff Garin, found JFK to be, by a wide margin, the most esteemed president since 1953 — a striking finding given Kennedy’s modest record of legislative achievement in office. Even more remarkable, his appeal transcends ideology: Fifty-two percent of Republicans and 79 percent of Democrats in Hart and Garin’s poll called him one of America’s best leaders. By contrast, other strong finishers, such as Ronald Reagan and Bill Clinton, are deeply disliked by members of the opposite party.

Kennedy’s enduring hold over Americans of all political stripes is the theme Sabato uses to unite what are, in effect, three separate books sharing a pair of covers. The first section of “The Kennedy Half-Century,” about 130 pages, is a serviceable history of JFK’s career. Designed to set the stage for Sabato’s discussion of the five decades since the assassination, this extended prologue makes no pretense of offering a new interpretation of JFK’s presidency. That’s a legitimate choice in a book like this, whose purposes lie elsewhere, but without a new angle or any fresh insight, this first section can’t justify its considerable length. For a primer on JFK’s life and career, readers would do better with any number of recent biographies, such as Robert Dallek’s “An Unfinished Life” or Richard Reeves’s “President Kennedy: Profile of Power.”

While the book’s first section is perfunctory, the second part, which deals with the assassination, is somewhat wearying and likely to interest only those hard-core buffs — I realize there are many — who wallow in outré speculation about who was behind Kennedy’s murder. For many years, this monomania was a widespread popular pastime, and for understandable reasons: the sheer horror of the violence; the sudden loss of a young, dynamic, activist president; and the shared sense that as Lyndon Johnson’s presidency wore on, the decade’s reformist energies somehow went awry, raising painful questions of what might have been. But if the political and cultural impact of the assassination remains worthy of exploration, drawn-out ruminations on the details of the deed will probably strike most readers as not worth their time.

The most promising section of Sabato’s book — its heart — is the third part, which methodically reviews how presidents from Johnson through Barack Obama have made use of JFK’s legacy for their own ends. Sabato isn’t the first historian to attempt this sort of study. His account owes a clear debt to Paul Henggeler, whose book “The Kennedy Persuasion: The Politics of Style Since JFK” had a similar approach, as well as to other scholars who have explored what historian Alan Brinkley called, in his own essay on the topic, Kennedy’s “posthumous lives.” Looming in the background of Sabato’s project, too, is William Leuchtenburg’s “In the Shadow of FDR,” a history of Roosevelt’s continuing influence on the presidents who followed him.

Some of the Kennedy mantle-claiming that Sabato reviews will be familiar to readers. Many historians have recounted how Johnson, days after JFK’s death, challenged the nation to honor its slain leader’s life by enacting his agenda: the economic program, the historic civil rights bill, the building blocks of what would become the Great Society. Well-known, too, is the account of Clinton as a presidential aspirant brandishing — almost as a totem, a corporeal link to Camelot — the picture of his 1963 handshake with JFK, taken when the 17-year-old Arkansan visited the White House as part of the Boys Nation youth group. Obama’s own seizing of the Kennedy torch — encapsulated in the moment he nabbed endorsements from Ted and Caroline Kennedy during his primary fight against Hillary Rodham Clinton in 2008 — also attests to the persistence of what Henggeler usefully called the “Kennedy mystique.”

The most original part of Sabato’s book may be its contention that Republicans now wrap themselves in Kennedy’s legacy almost as much as Democrats do. And it’s true: Alongside the poll numbers showing Republicans’ admiration for JFK, Sabato provides evidence of how assiduously Reagan and his aides sought to appropriate the Kennedy luster. Under Reagan, the Republican National Committee even compiled a Kennedy “quote file” that administration officials could use to argue for conservative policies, and the president himself, in pushing to lower the high marginal income tax rate, repeatedly noted that JFK had tried to do the same.

Such rhetoric, of course, rested on misreadings, naive or deliberate. Reagan’s tax cuts were supply-side cuts, designed to put money in the hands of the wealthy, who were then expected to invest it. In contrast, the 1964 Revenue Act, as Arthur Okun, one of Kennedy’s economic advisers, explained, “was aimed at the demand, rather than the supply, side of the economy.” The Kennedy plan did cut the top marginal income tax rates, but its principles were Keynesian, aimed at giving money to a broad swath of citizens who would rapidly spend it, thereby spurring economic activity.

The Reaganites also misread Kennedy’s record in reinventing him as a hawkish Cold Warrior. It has become common to cite the “pay any price, bear any burden” line of Kennedy’s inaugural address to argue that he wanted to hunker down for “a long twilight struggle.” But while Kennedy certainly had no illusions about the communist menace, this revisionist account of his speech omits his courageous calls for arms control, aid to the Third World and collaborations with the Soviet Union in science and other areas. The Washington Post’s headline the next day read, “Kennedy Takes Oath as President, Proclaims a New ‘Quest for Peace.’ ” The politicization of Kennedy as a hawk, however, only strengthens the thesis that he wields uncommon power as a rhetorical symbol.

That JFK’s successors have deployed his name and words so frequently — and that Kennedy has, as Sabato notes, transcended partisanship — is nonetheless something of a riddle in need of answering. After all, the other presidents whom politicians love most to invoke, Abraham Lincoln and Franklin Roosevelt, both led the United States through crises that transformed the nation. Kennedy, his sure-footed handling of the Cuban missile crisis notwithstanding, can claim no such feat.

Sabato chalks up the lasting Kennedy mystique to a combination of “powerful optics” and “genuine inspiration.” Those explanations are sound, as far as they go, but not sufficient. To dig deeper, Sabato might have spent more time, ironically, on the assassination — not on the arcana of the grassy knoll and the Fair Play for Cuba Committee, but on the reasons Kennedy’s bright presidency and cruelly curtailed life became, after November 1963, a focal point for all that went wrong in the late 1960s — Vietnam, riots, a loss of trust in government — and a repository for the dreams of what might have been.

David Greenberg ,, a professor of history and of journalism and media studies at Rutgers University, is the author of “Nixon’s Shadow: The History of an Image,” among other works. He is writing a history of presidential spin.

More books on Kennedy:

What the Warren Commission missed

Kennedy got a lot of bad advice

The murder of a husband and father

What was Oswald’s motive?

What if Kennedy had lived?

Jim Lehrer imagines Secret Service guilt

A roundup of Kennedy books


The Presidency, Assassination, and Lasting Legacy of John F. Kennedy

By Larry J. Sabato

Bloomsbury. 603 pp. $30.