Generations from now, a historical oddity will demand explanation. On children's placemats, and in textbook lists ready for memorization, Barack Obama and Donald Trump will reside as neighbors.
The 44th and 45th presidents of the United States may be "sort of opposites," as the outgoing president put it in an interview last week. History, though, will link them forever -- a fact that was on Obama's mind as he delivered his farewell address at Chicago's McCormick Place Tuesday night.
"Yes, our progress has been uneven," the president said. "For every two steps forward, it often feels we take one step back."
Obama offered praise for "the instrument of our democracy." Yet it is democracy's result that leaves his legacy in the balance. And challenges to American democracy are occupying the outgoing president's brain space in his final days in office.
For Obama and his supporters, a stubborn fact will linger long into the Trump era. Donald Trump would not exist as a political entity if not for Barack Obama. His elevation to the presidency would not have been possible without a backlash against the president whose time is coming to a close.
That means, for a good stretch of the Trump presidency if not longer, Trump's success will be defined by his ability to undo what Obama did. The Trump whirlwind is already overtaking Obama, and January 20 will begin a countdown toward the unraveling of key initiatives regarding immigration, the climate, and race relations.
Legislation and judicial appointments are likely to further scale back key achievements -- starting, most notably, with Obamacare. The hearings for Trump nominees taking place this week only hint at the pent-up conservative demand for a radically different direction.
In response to the new reality, Obama is seeking to leave office with reminders of what got him there. With his calls to accept diversity and elevate political discourse, he set that up as a veiled warning of what his successor might bring.
"If we don't create opportunity for all people," he said Tuesday night, "the disaffection and division that has stalled our progress will only sharpen in years to come."
Obama leaves office with an array of achievements -- in health care, social justice, the economy, the environment, and efforts to disentangle the nation from a long period of wars. Yet the list of unfinished business is always and inarguably longer.
A Hillary Clinton presidency would have sealed many Obama achievements for a generation or more, particularly with the president's ability to shape the Supreme Court. The surprise in the other direction, though, leaves virtually everything Obama achieved vulnerable.
Predictably, the hometown crowd in Chicago broke out into chants -- "four more years!"
"I can't do that," Obama responded.
As he exits, what he hopes will remain is what he represented, and will continue to represent, when future children look at their place mats.
"You were the change," the president said.
"Yes we can," he said in closing. "Yes we did. Yes we can."
ANALYSIS: For Obama, a Legacy Trump Will Help Define