WASHINGTON -- Along the National Mall in Washington, D.C., off Independence Avenue near 7th Street, there once stood a building known as the Yellow House. According to Jesse Holland's "Black Men Built the Capitol," it appeared from the outside like other dwellings. In the basement, with iron bars on the windows and rings in the floor for chains, and in a yard enclosed by a 12-foot wall, enslaved human beings were kept and sold.

One of them was Solomon Northup, a free black man from New York who was kidnapped, sold into slavery and imprisoned at the Yellow House. He later wrote, "Strange as it may seem, within plain sight of this same house, looking down from its commanding height upon it, was the Capitol. The voices of patriotic representatives boasting of freedom and equality, and the rattling of the poor slave's chains, almost commingled."

In a few days, President Barack Obama's voice will mingle with those ghostly sounds, and be added to others. Marian Anderson singing "My Country, 'Tis of Thee," though the Daughters of the American Revolution had mocked that hymn's premise. Martin Luther King Jr. speaking on the steps of the Lincoln Memorial, where a small plaque now marks a holy place of American rhetoric.

If Obama and his talented speechwriter, Jon Favreau, cannot find poetry in this place, they will never find it.

No doubt they will. But there are enemies of poetry and ambition in every speechwriting process. The political advisers who chant, "It's the economy, stupid." The focus group disciples who explain that the dial groups don't like the words "slavery" or "injustice"; they prefer words such as "buttercup" and "marshmallow." The communication consultants who use "rhetorical" as a pejorative because formality doesn't play well "around the kitchen table." Especially in producing an inaugural address, all of them must be ignored. It is appropriate to mention current events, but the State of the Union allows specificity soon enough. An inaugural presents different tests for a new president: Can he stop talking like a candidate, and speak for the country and its purposes? Can he place his barely started chapter in the context of the American story?

That story has many themes, but one major challenge: a desperate, sometimes bloody, search for unity. The consequential inaugurals confront the issue directly. Presidents before Lincoln attempted to maintain a political union of fractious states. Once shattered, warned Franklin Pierce, "no earthly power or wisdom could ever reunite its broken fragments." Lincoln set out the ideal of a spiritual union -- a union of idealism and of shared suffering -- that transcended race and took a century to even partially achieve. Presidents in the tradition of Woodrow Wilson and Franklin Roosevelt asserted a national unity founded on democratic idealism, in a world gone mad from imperialism, racism and ideology. "The democratic aspiration is no mere recent phase in human history," said FDR in his third inaugural. "It is human history."

In an earlier inaugural speech, Roosevelt observed, "In every land there are always at work forces that drive men apart and forces that draw men together." Those forces remain at work. Through the tumults of the '60s and '70s, America experienced divisions that turned generation against generation. Today our cultural and political differences seem mainly expressed by derision, in a kind of spiritual secession from one another.

It is the primary purpose of presidential leadership to be a force that draws us together -- to declare, as Jefferson did, that we are "brethren of the same principle," to state and plead, as Lincoln did, that "We are not enemies, but friends. We must not be enemies." The origin of this unity for Americans is not an accident of blood or birth, but certain shared moral affirmations about the rights and dignity of all men and women -- assertions contained in the Declaration of Independence, and uncontained in their global influence. The existence of those rights imposes duties on government -- and creates obligations of citizens to each other.

In an inaugural address equal to his moment, Obama will summarize a historical achievement he already symbolizes -- and explain how this flawed, grand, God-shaken story moves forward to include everyone. This hope of unity is stronger than all the hypocrisy of our past, and louder than the clank of chains. It led men and women to travel on immigrant ships and the Underground Railroad -- and it explains the amazing journey from the Yellow House to a white one just down the street.