Barack Obama will retire a president personally popular with the American people yet who served them (and himself, and his party) badly.

He fretted in 2012 that he would lose the election just in time for Mitt Romney to get credit for an Obama recovery. That long-delayed recovery is finally coming in the last months of his administration—the economy finally broke 3% growth in the third quarter—and now Mr. Trump will get the credit.

He...