Donald Trump revived one of his oldest political attack lines this week when he derided Kamala Harris as a 'low IQ person' while musing aloud about whether egg producers could ever have backed her. The remark, captured in a widely circulated video clip, was brief but revealing. It fused two themes that have become staples of Trump's political style: personal insult as campaign shorthand, and an instinct to turn even a narrow economic grievance –…