Being martyred confers certain privileges, the first being forgiven your sins by those who loved you.

Alas, many who loved JFK covered JFK, and covered up his many failures and foibles. To this day, JFK’s presidency is ranked high by most historians.

A new book sets out to burnish his record yet again, but fails, writes Frank Gannon.

…”JFK’s Last Hundred Days” is a superb piece of writing—richly detailed and, considering that the end is all too well known, surprisingly enthralling. It’s too bad that Mr. Clarke is burdened by a thesis that is often belied by his material. Of course, it is possible that in his second term JFK would have championed all the causes that prudence and realpolitik had forced him to soft-pedal during his first four years. But a skeptic might say that old problems were lurking just below the surface during the period when Mr. Clarke sees a great president emerging.

Most glaringly, Kennedy had approved a memo that gave the green light to a military coup against South Vietnam’s President Diem, and although Mr. Clarke cuts him every kind of slack, he has to admit that, by doing so, Kennedy was violating one of the “lessons of his presidency”—to avoid precipitate decisions. Kennedy might have quit Vietnam in his second term; it’s equally possible that the arguments of his hawkish advisers or unfolding events there—a coup isn’t the most stabilizing geopolitical strategy—would have led him to extend his commitments.

During these final months, as Mr. Clarke acknowledges, the president was sitting atop “a Greenland-sized mass of secrets and subterfuge that included his frantic philandering, the White House taping system, and his perilous health.” The revelation of any of these during the 1964 campaign might have been devastating. Despite Kennedy’s intentions to control himself, he invited one of his mistresses for a 7-hour visit to the White House in October, while the first lady was away. That Barry Goldwater was personally upright meant that JFK couldn’t count on a continuing conspiracy of silence over his own profligate life. With another opponent, the rules might have been different. As Kennedy put it in one of Camelot’s less elevated moments: “I just figure that a guy who’s getting laid is not going to go after a guy who’s getting laid.”

Mr. Clarke tries to find reasons for Kennedy’s compulsive womanizing: that he couldn’t tolerate being bored; that his wartime brush with death left him addicted to risk; that the steroids he took for Addison’s disease supercharged his libido; that his father had twisted him. Maybe so. But they don’t do much to make the case for a remoralized president on the cusp of greatness.

Five hours after the test-ban treaty was signed, Mr. Clarke notes, “an event occurred that threatened to end his presidency in impeachment and disgrace.” This was the Bobby Baker scandal. Baker was Lyndon Johnson’s Senate fixer; an investigation of his corrupt activities would have exposed his role as one of Capitol Hill’s premier procurers and revealed JFK’s liaison with Ellen Rometsch, who was suspected of being an East German spy.

Robert Kennedy had had Rometsch deported, but her absence wouldn’t keep investigators from bringing her activities to light. So the president reached out to his erstwhile nemesis, J. Edgar Hoover, who persuaded Senate leaders that Rometsch and Baker’s other party girls were off-limits to Senate hearings.

As Mr. Clarke shows, the president was also feeling vulnerable about his Cold War credentials, making the Rometsch affair all the more toxic…