GETTYSBURG, Pa. — “Lincoln hated Thomas Jefferson.” That is not exactly what we expect to hear about the president who spoke of “malice toward none,” referring to the president who wrote that “all men are created equal.” Presidents have never been immune from criticism by other presidents. But Jefferson and Lincoln? These two stare down at us from Mount Rushmore as heroic, stainless and serene, and any suggestion of disharmony seems somehow a criticism of America itself. Still, Lincoln seems not to have gotten that message. “Mr. Lincoln hated Thomas Jefferson as a man,” wrote William Henry Herndon, Lincoln’s law...