Of the 37 counts leveled at President Trump in Special Counsel Jack Smith’s indictment, 31 of them relate to the Espionage Act, telegraphing that the case against Mr. Trump is likely to double as a turn in the spotlight for a law that many dissidents and radicals have loathed for more than a century. Now, if a jury determines that Mr. Trump acted crosswise with its provisions, he could go to prison.