WASHINGTON — Since the Civil War, the majestic dome of the U.S. Capitol has symbolized the unity of the United States, despite the discord in the government it overlooks.

Abraham Lincoln is said to have urged the continuation of its construction during the war, saying it showed that the Union would endure.

The poet Walt Whitman, who wandered Washington at night, wrote of the dome: “it comforts me somehow.”

But the dome has lately grown as fractured as the federal government, and Tuesday the Architect of the Capitol announced that a $59 million project to save it would begin next month.

The multi-year project will include covering the entire dome in scaffolding, from the base of the statue of Freedom, which sits on top, to the “skirt” that rings the bottom, where some work was done last year.

The dome would join the Washington Monument as another Washington landmark covered in scaffolding.

A white, doughnut-shaped protective canopy of plastic netting will be installed in the interior of the dome to guard against falling debris. Scaffold towers and bridging will also go up the west side of the Capitol to help move materials.

“The last significant exterior renovation of the dome was performed in 1959-60, when it was stripped of its paint so the ironwork could be repaired and primed with a rust inhibitor,” the architect’s office said in a statement.

Water has seeped through holes and cracks in the dome, rusting ironwork and damaging some decorative metal elements, the statement said.

There are now hundreds of cracks and deficiencies, and water already has stained parts of the Capitol’s interior. Some heavily deteriorated metal decorations that were in danger of falling off the exterior were discovered and removed during inspections. One weighed 40 pounds.

More than 100 such pieces were found and removed, Carlos Elias, superintendent of the Capitol building, said in a video accompanying the announcement. He said they would try to reattach or replicate the originals.

Most of the dome is made of cast iron. The concern is that continued water leaks could further damage the interior of the Capitol beneath the dome. In addition, “the safety of visitors to the Capitol is threatened by falling objects,” the architect’s statement said. The project plans to restore cast-iron elements, weatherproof the exterior, replace broken windows, and remove hazardous materials, the architect’s statement said.

“As people drive by this capitol building today, it really looks pristine and magnificent,” Architect of the Capitol Stephen Ayers said in the video. “But when you get up close you can see with your own eye how deteriorated the building and the cast-iron work actually is.”

Ayers said experts have been watching the deterioration for a decade, and it is now time to take action.

Kevin Hildebrand, head of the office’s architectural branch, said there are “almost 1,300 known cracks and breaks in the exterior skin of the dome.”

Most of the repair work will be done at night and on weekends to minimize disruption to congressional business, events and tours.

This year is the 150th anniversary of the completion of the current dome. It was under construction when Lincoln was first sworn in at the Capitol in 1861. It was finished when the final section of the statue of Freedom was lifted to the top on Dec. 2, 1863.