WASHINGTON — Fifty years after John F. Kennedy’s assassination, he remains an object of almost universal admiration. And yet, particularly this year, his legacy has aroused the ire of debunkers who complain that Kennedy is unworthy of all this adulation.

“John F. Kennedy probably was the worst American president of the previous century,” wrote the journalist Thomas E. Ricks. “He spent his 35 months in the White House stumbling from crisis to fiasco.”

He was, they say, all image and no substance, a shallow playboy whose foreign policy mistakes and paltry legislative record undermine any claim to greatness. His assassination, personal attributes of good looks and charm, joined to Jacqueline Kennedy’s promotion of a Camelot myth, have gone far to explain his popularity.

Such criticism not only gives short shrift to Kennedy’s real achievements as a domestic and foreign policy leader, but it also fails to appreciate the presidency’s central role: to inspire and encourage the country to move forward, a role that Kennedy performed better than any president in modern memory....