President Trump, in clumsy responses to interview questions, has warned Special Counsel Robert Mueller that he’d best not expand his investigation of Russian interference in the 2016 election too far. It is strange, to put it no more strongly, to read such threats against the investigator by a president whose adminisitration is being investigated. It carries weight, for Trump has the authority to fire Mueller, but to do so would do incalculable damage to his presidency.