One the US Attorney especially in the larger districts rarely if ever prosecutes cases personally. That is the job of Assistant US Attorneys.
It’s a management position, he may formally sign the complaints but typically will not try them. His responsibility is to set policy, assign and supervise the work of his subordinates and kept apprised on the progress of trials. What the US Attorney may do when necessary is decide trial strategy including when to indict, sue, settle cases etc. He is responsible for what goes on in that office and in a sense they are all his trials Franken knows this and to get into this type of chickenshit gotcha doesn’t say much for his questioning.