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 on 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.