“I don’t have but one speech,” Martin Luther King, Jr., once said. “I don’t have but one message as I journey around this country.” Few of King’s contemporaries took this notion to heart more fully than Ronald Reagan. Beginning in 1954, when Reagan became, in effect, the in-house motivational speaker for General Electric, he delivered, many hundreds of times, what was known as “The Speech.” From plant to plant, from one year to the next, Reagan honed his script, reshuffled his note cards, and updated his anecdotes, but his theme—the threat of an encroaching, expanding government—did not vary. It was less a speech than a sermon, as Reagan himself understood—a malediction against the evils of income taxes, federal spending, central planning, godless Communism, and government controls on commerce and freedom. “We’ll preserve for our children this, the last best hope of man on Earth, or we’ll sentence them to take the last step into a thousand years of darkness,” Reagan said. If there is such a thing as a feel-good jeremiad, Reagan invented it.

The Speech was the speech that Reagan gave on prime-time television on October 27, 1964—fifty years ago this week—as a campaign address in support of Barry Goldwater, the Republican Presidential nominee. It accomplished less for Goldwater than it did for Reagan himself; it propelled him, immediately and justifiably, into a kind of stardom that he had never quite achieved in Hollywood. That half hour on NBC did for Reagan and modern conservatism what “The Ed Sullivan Show,” six months earlier, had done for the Beatles, rock music, and popular culture: it drew a bright line between the past and the future. The Speech—rechristened, for that occasion, as “A Time for Choosing”—helped to define the G.O.P. and conservative politics for more than a generation.

That is the message, anyway, this anniversary week—in op-eds affirming that The Speech is “still great,” in lectures, in public screenings, and in a celebratory event at Rancho del Cielo, Reagan’s home in the Santa Ynez Mountains, that featured, as the Washington Times reported, “Fox News regulars” and other speakers, who were on tap to “parse the speech for enthusiastic young audiences.” These commemorations are not so different in tone from last year’s remembrance of the March on Washington, where King gave his great speech, in 1963. But Reagan remains, for Republicans, a monomaniacal obsession—someone to be celebrated and cited and emulated and impersonated without regard to the calendar and, indeed, without cease.

The singularity of Reagan and his lonely place in the conservative pantheon is put in stark relief by photographs of the 1964 Democratic National Convention, in Atlantic City, where massive portraits of Franklin Roosevelt, Harry Truman, John F. Kennedy, and Lyndon Johnson framed the stage. It is impossible to imagine a similar setup at the Republican Convention in 2016. Other than Reagan’s, whose image—among the past century’s Republican Presidents—would be put on display? Coolidge has a cult following (which included Reagan himself); Eisenhower has supporters, but also serious detractors in the Party’s right wing (today as in the nineteen-fifties); George H. W. Bush has garnered enough goodwill and retrospective credit in the years since his Presidency that he might merit inclusion; but none of these men really stir the blood. “Nixon’s the One,” proclaimed bumper stickers and buttons in 1968, but this was only wishful thinking. Reagan was already the one, even if America didn’t know it yet.

Reagan’s appeal seems only to intensify with time. A decade after his death, almost twenty years to the day after his disappearance into the fog of Alzheimers disease—“the journey that will lead me into the sunset of my life,” as he described it in a farewell letter—commentators continue to set forth the “true meaning of Reaganism” with the fervor that leftist intellectuals once expounded the meaning of Marxism, and a rising generation of Republicans is staking its claims on Reagan’s legacy. Senator Marco Rubio cites Reagan on everything from immigration reform to defense to the future of the space program; Senator Lindsey Graham, a fan, calls Rubio “the son of Ronald Reagan when it comes to national security.” Rubio’s potential rival for the Presidential nomination, Senator Ted Cruz, is also grabbing at the mantle. “Mr. Putin, give back Crimea,” he demanded in July. “TED CRUZ CHANNELS REAGAN,” Business Insider explained, in case you’ve forgotten Mr. Gorbachev and the Wall...