Barack Obama has always evinced a fascination with history. He announced his candidacy in Springfield, Illinois, recalling Abraham Lincoln. He modeled his own cabinet after Lincoln’s “team of rivals.” He has compared his own accomplishments to his predecessors, and he invited historians to the White House for private conversations about where he might fit within the pantheon of American leaders.

If Obama’s interests run toward history, so does his rhetoric. “It's the answer that led those who have been told for so long by so many to be cynical, and fearful, and doubtful of what we can achieve to put their hands on the arc of history and bend it once more toward the hope of a better day,” he said the evening of his first election. Since then, the president has repeatedly deployed a series of phrases—especially “the right side of history” and “the wrong side of history”—that suggest a tortured, idealistic, and ultimately untenable vision of what history is and how it works.

Most recently, during is December 6 Oval Office address on terrorism, Obama said: “My fellow Americans, I am confident we will succeed in this mission because we are on the right side of history.” It’s a phrase Obama loves: He’s used it 15 times, in debates; at synagogues; in weekly radio addresses; at fundraisers. Obama is almost as fond of its converse, “the wrong side of history,” which he has used 13 times; staffers and press secretaries have invoked it a further 16. (These figures are all based on the archives of the American Presidency Project at the University of California Santa Barbara.)

But the expressions are hardly original to Obama. Bill Clinton referred to “the right side of history” 21 times over his time in office, while his staffers added another 15. Clinton also mentioned the “wrong side of history” several times. Ronald Reagan, for his part, wryly resurrected Leon Trotsky’s relegation of the Mensheviks to the “dustbin” or “ash heap of history.” Speaking to the British Parliament in 1982, the Gipper said, “The march of freedom and democracy which will leave Marxism-Leninism on the ash-heap of history as it has left other tyrannies which stifle the freedom and muzzle the self-expression of the people.” Reagan used both translations of Trotsky’s phrase several more times.

Obama’s own fresh contribution to the genre is his invocation of “the arc of history.” It’s his adaptation of an older phrase, “The arc of the moral universe is long but it bends toward justice,” which was popularized by Martin Luther King Jr. but coined (evidently) a century earlier by Theodore Parker. Obama has mentioned “the arc of history” a dozen times since his election. ...