Fearing a floor revolt against his nomination, Joe Biden’s aides are telephoning individual delegates to next month’s Democratic convention to gauge their loyalty to the president, according to three delegates who received a call this week. After a round of introductory questions confirming each delegate was still planning on going to Chicago and asking if they had served as delegate before, the Biden aide making the calls got to the point, the Democratic activists recalled to me in separate interviews. One of the delegates said the line of questioning turned with an inquiry along the lines of, “Do you understand...