David W. Blight is the author of “American Oracle: The Civil War in the Civil Rights Era” and a professor of history at Yale, from which Allison Scharfstein is graduating this month.

ON May 17, 1962, the Rev. Dr. Martin Luther King Jr. delivered an extraordinary manifesto to the White House. Constructed as both a moral appeal and a legal brief, the 64-page document called on President John F. Kennedy to issue a “second Emancipation Proclamation,” an executive order outlawing segregation — just as President Abraham Lincoln had done with slavery a century earlier.

The civil rights era, like the Civil War, produced a wealth of great writing. But unlike King’s “I Have a Dream” speech or his “Letter From Birmingham Jail,” which we remember for their visionary rhetoric, this extraordinary document has been virtually forgotten.

And yet the manifesto is a wonderful example of King’s close reading of American politics, as well as his understanding of the role that moral leadership, in this case through an executive order, could have on the American public. It’s a lesson we should take to heart today, when a deadlocked Congress stands in contrast to a president willing to take a bold stand on same-sex marriage. Americans have rarely explicitly voted for equality; history, through institutions and a few courageous leaders, has enacted it....