There are two words for those who think the US republic is teetering: Donald Trump. There is also a two-word riposte: Robert Mueller. While Mr Mueller stays in his job, the US system is working as it should. Credit where it is due. The single-minded former Federal Bureau of Investigation chief was appointed as special counsel by Mr Trump’s administration. If a nation’s politics is judged on how it responds to alleged crime in high places, America is acting in textbook fashion. In the words of his biographer, Mr Mueller is “America’s straightest arrow”. He is entrusted with the integrity of the system.

Yet the temptation for Mr Trump to fire him grows by the day. It would be hard to think of anyone more inoculated against a smear campaign than Mr Mueller. Unlike many of his detractors, he fought in the Vietnam war. He was wounded but voluntarily returned to lead his platoon in combat. He won several medals, including a Purple Heart. At the FBI he oversaw the largest mafia investigation in history, which ended with the conviction of John Gotti, head of the powerful Gambino family. A registered Republican, he was appointed to head the FBI by George W Bush — a job he began one week before the 11 September attacks.

Such a record tells you two things. Mr Mueller is an all-American hero and he will not be intimidated. He cuts the kind of figure Mr Trump normally admires — a tall, square-jawed, patriot straight from central casting. That, of course, is why Mr Trump wants to sack him. Mr Mueller has that zealous G-man quality that sends shivers down White House spines. Whatever Mr Mueller’s investigation turns up — anyone who thinks they know where it will end is bluffing — the process is unflinching. Ask Paul Manafort, Mr Trump’s former campaign manager, who was marched into court on Monday to hear 12 criminal counts against him. Or George Papadopoulos, Mr Trump’s former campaign adviser, who has pleaded guilty to lying to the FBI. Here is a public servant who actually believes in public service.