Louis P. Masur is a professor of American studies and history at Rutgers University and the author of "Lincoln's Hundred Days: The Emancipation Proclamation and the War for the Union."

A couple of years ago, speaking to a bipartisan group of college students about the Emancipation Proclamation, President Barack Obama commented, half jokingly, that if the executive order were signed today, headlines would scream, “Lincoln Sells Out Slaves.” His observation spoke not only to our sensationalist news culture, but also to the rocky reputation of the Proclamation itself, a document that has been both praised and damned by politicians, scholars, and activists on both sides of the ideological aisle since Lincoln announced it in 1862 and then signed it 150 years ago this year, on January 1, 1863.

The reasons behind the ups and downs in the Proclamation’s reputation are various. From the outset, it was roundly and predictably condemned by Democratic opponents, who characterized it as a brash and sweeping abuse of presidential power. Perhaps less predictably, Northern abolitionists also condemned it, but for the opposite reason. They argued that it didn’t do enough, didn’t go far enough. Since the Emancipation Proclamation applied only to those slaves in rebel-held territory, abolitionists complained that it abandoned thousands of slaves, including the four loyal slave states: Delaware, Maryland, Missouri, and Kentucky. Of the four million slaves in America at the time, the Proclamation applied to only about 3.1 million of them. It would take another three years and the ratification of the Thirteenth Amendment in December 1865 to abolish slavery throughout the United States. Adam Gurowski, a Polish radical working as a translator in the State Department at the time, despaired that “the proclamation is generated neither by Lincoln’s brains, heart or soul, and what is born in such a way is always monstrous.”

Despite Gurowski’s prediction, in the decades following the Civil War, until the middle of the twentieth century, the reputation of the Emancipation Proclamation expanded in most Americans’ estimation, reaching a rather exalted status in the American history textbooks in many of our childhoods. But then, in the 1960s, the Proclamation’s reputation began to shrivel again. Some historians began to find the prose wanting. It irked them that the Proclamation was written in legalese as a military measure, not as an expression of moral conviction—evidence, they thought, that the document was “merely” the product of political calculation and compromise. Lincoln was found wanting, too. Steeped in the realities of the nineteenth century, Lincoln’s racial attitudes seemed out of step with the times....