You introduce yourself to voters as a son of Kansas and Kenya, an emblem of this country’s openness to outsiders and its embrace of difference. Your election and re-election affirm the distance that the United States has traveled, or so you believe. So you hope.

Then you look up toward the end of your second term to behold a Republican presidential nominee who is cynically exploiting racism and xenophobia to put the White House within his own reach. He’s not merely your adversary; he’s your antithesis. And his victory would do more than endanger your policies. It would question the very moral of your journey, the very bend of the arc you frequently invoke.

That’s what Barack Obama confronts right now, and that’s why he hit the campaign trail Tuesday, appearing onstage with Hillary Clinton in North Carolina and proclaiming without reservation that “there has never been any man or woman more qualified for this office” than she. That’s why he’ll say words like those again and again, with the same fire, in the months ahead.

For the nation’s first black president, Clinton isn’t just the better candidate. She’s the better America. She wins and he holds on to his rosiest convictions about what he and his presidency symbolize. Donald Trump wins and that’s a tricky thing to do.

Trump forged his bond with bigots by essentially calling Obama an impostor and demanding to see his birth certificate. But that particular stunt weighs less on Obama than Trump’s sustained behavior during the 2016 presidential race does, according to people close to the president.

“The thing that I’m sure aggravates him — enrages him — is the invocation of race and ethnicity in our politics,” David Axelrod, a former White House aide, told me. “Obama’s message is about the emerging America and the strength of our diversity. He represents it. And when Trump says ‘Make America great again,’ there’s an element of turning the clock back to the days when minorities were at the back of the bus.”

“That goes to the character of our country,” Axelrod added. “The president is someone who would be uniquely sensitive to that.”

Uniquely sensitive and utterly impassioned. In North Carolina he didn’t so much urge voters as command them, with a testimonial about Clinton that was gushing and epic. I swear I saw her blush.

Was Trump on Obama’s mind? I suspect. “Everybody can tweet,” he said, adding that it’s no preparation or qualification for the presidency. He brought up his younger daughter. “Sasha tweets, but she doesn’t think that she thereby should be sitting behind the desk.”

Was Trump on Clinton’s mind? Clearly. She complimented Obama as “someone who has never forgotten where he came from — and Donald, if you’re out there tweeting, it’s Hawaii.”

The 2016 campaign keeps showing us things that we’re not accustomed to, and a second-term president campaigning with unfettered vigor for his desired successor is another of those. George W. Bush didn’t do it: He was so toxic at this point in his administration that John McCain’s most fervent wish was to tuck him into a broom closet.

Bill Clinton didn’t do it, because Al Gore was intent on coming across as his own, less priapic man. Neither did Ronald Reagan, because Bush’s father similarly felt the need to flex his own muscle, outside of anyone’s shadow, and Reagan’s energy was flagging anyway.

Dwight Eisenhower? When asked what Richard Nixon had accomplished as his vice president, he said that he needed a week to think about it.

Obama and Hillary Clinton have arrived at a place of obvious respect for each other, and of palpable fondness. His high approval ratings put him in a position to help. Her stature puts her in a position not to be eclipsed by his presence or belittled by that assistance.

Campaigning together is an imperfect arrangement, inasmuch as she may seem to be arguing for the status quo instead of a better tomorrow. But Americans hold Obama in significantly higher esteem than they do her or Trump. There are far riskier things than letting the president carry the ball.

And he’s a player in this regardless, given the larger context, which was clear when Clinton asked the North Carolina audience to think of “the early patriots who met in Philadelphia” in 1776.

“Nobody who looked like Barack Obama or me would have been included back then,” she said. “But we’re here today because the story of America is the story of hard-fought, hard-won progress.”

That’s the tale that Obama has always told. It’s the narrative that so many of us cling to. Where does Trump fit into it, and does it survive him? Instead of just wondering and worrying, the departing president has joined the fight.

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Frank Bruni is a New York Times columnist.

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