The Prime Minister clammed up on the Today programme this week when asked to confirm reports that Coulson had offered to resign to end the difficulty that the ongoing phone-hacking scandal is causing.
A man who usually sounds at such ease in front of a microphone suddenly found himself bluntly refusing to disclose 'private conversations'.
Cameron is right to be on edge: he knows that the darker the clouds gathering above the former News of the World editor become, the bigger the question marks over his own judgement. Because when the Tory leader hired Coulson, he did not attempt simply to forgive the controversy in his past. By saying he had fully accepted assurances that Coulson knew nothing whatsoever about the illegal activity that went on during his time in charge, Cameron tied his own credibility to his employee's integrity from the outset.
No politician can ultimately survive without trust. Even though the media have been highly reticent to put the boot in, Cameron will worry that the public's trust in him is slightly eroded every time they hear him say he believes without doubt something that so many find simply incredible. So, this week, the Prime Minister notably chose not to repeat his expression of faith that Coulson neither saw nor heard any evil from his editor's desk. Instead, he simply said that his comms chief deserved a second chance.
That is significant, as it shows it must now be finely balanced whether the damage this is causing outweighs the adverse reputational impact of letting him go and losing the talents of someone of whom journalists generally speak highly.
You do not have to be a veteran of the Downing Street from which Damian McBride was expelled to realise that it is not a happy place for a Prime Minister to be.
John Woodcock is Labour MP for Barrow and Furness and a former spokesman for ex-prime minister Gordon Brown