As the Government's new Chief Whip, Andrew "Thrasher" Mitchell's task is to inject discipline into the fractious ranks of Tory MPs frustrated with the compromises of coalition. But his astonishing outburst at the police officers guarding the Downing Street gates reveals an embarrassing lack of self-restraint by the man charged with knocking his backbenchers into shape.
And any episode that reinforces perceptions of a Conservative leadership dominated by a narrow and privileged social circle has the potential to inflict lasting damage on the party.
It is also a wounding personal blow to Mr Mitchell, who worked hard, with his charity wristbands and regular trips to Rwanda, to reinvent his political image during his two-year spell as International Development Secretary.
Originally on the party's right – he ran David Davis's ill-fated leadership campaign against Mr Cameron in 2005 – he became close to the Prime Minister and later turned into a passionate advocate of coalition government.
Mr Cameron rewarded him in this month's reshuffle with the key post of Chief Whip, partly in response to a reputation for an authoritarian streak which dates back decades. Mr Mitchell admits to being a "stern disciplinarian" from his time as a prefect at Rugby School, although he credits Private Eye with coining the nickname.
According to one Westminster rumour, he is known to some special advisers as "BSD" after telling DfID officials he was a "big swinging d***".
Mr Mitchell is the fourth generation of his family to serve as an MP, and has the quintessential Tory background: Cambridge, a spell in the Army and merchant banking before arriving in the Commons aged just 31.
The only blip came when he lost his Nottinghamshire seat in the Blair landslide, but he returned four years later as MP for Sutton Coldfield. One of the wealthier members of a wealthy Cabinet, he divides his time between a London townhouse, a constituency home and a villa in the ski resort of Val d'Isere.