The Civil War exhibit that opened last month at the Library of Congress will gain a special addition on Jan. 3, when curators for the first time in almost four years will display President Abraham Lincoln’s handwritten first draft of the Emancipation Proclamation.

The seismic document freed thousands of slaves and paved the way for passage of the 13th Amendment, which permanently abolished slavery. The proclamation will be seen for six weeks in the southwest exhibition gallery of the library’s Thomas Jefferson Building to coincide with the 150th anniversary of its signing on Jan. 1, 1863.

For all its significance, the document divided Lincoln’s Cabinet, coming ahead of midterm elections and while the United States was achieving only mixed success against Confederate forces.

It pledged to free slaves in all the states still in active rebellion, thus depriving the South of a working population that allowed it to mobilize most of its white, military-aged males.

Lincoln read the draft to his Cabinet on July 22, 1862, but was persuaded to hold off formally presenting the proclamation until the Union army scored a significant victory. That came in September at Antietam.

Lincoln justified the document as a war measure that weakened the enemy by taking away its labor force, according to Michelle Krowl, a Civil War specialist in the library’s manuscript division. That was an important bit of framing because many Northerners viewed the war as primarily a struggle for union, not freedom.

The document will join more than 200 items on display in the library’s “The Civil War in America” exhibit, including dozens of letters and diaries from individuals who lived through the conflict, a large map of Virginia’s Shenandoah Valley commissioned by Gen. Stonewall Jackson and some of Lincoln’s personal items, including the contents of his pockets the night he was assassinated.

The exhibit, which has been viewed by more than 40,000 visitors since opening Nov. 12, runs through June 1. The Emancipation Proclamation draft, which was last on public view in 2009, will be on display through Feb. 18. Hours are 8:30 a.m. to 4:30 p.m. Monday through Saturday.