A week, 10 days, a fortnight - even the doyen of political spin can’t quite remember his own golden rule.
But, as the fraught debate on the debates enters its sixth day (since Ofcom decided the Greens couldn’t be treated as a major party at the general election), many will suspect the pressure on David Cameron to concede to the prevailing mood has become unbearable.
Put simply, the Prime Minister and his closest advisers would be mad to cave.
Some of the reasons are obvious.
Cameron is way out in front of all the other party leaders in polling on leadership and statesmanship – ground he concedes by standing on a platform beside Ed Miliband and two people who can never be PM.
The Labour leader, who in the popular imagination sits somewhere between a muddled Chuckle Brother and Wallace (of ‘& Gromit’ fame), would gain even a minor boost by managing to remember the countries that make up the UK, let alone its single biggest issue.
Nigel Farage will adopt the role of the plucky outsider vacated by Nick Clegg circa 2010 – dragging Cameron to his right (deadly), or peeling off those Conservative voters who subscribe to the ‘this country ain’t what it used to be’ school of thought.
All of this will occur while the leader of the Lib ‘differentiation’ Dems buddies up with Miliband to bemoan the PM as a heartless toff committed to helping only those who did their Duke of Edinburgh Award.
It’s a no-win situation for Dave.
Yet, at the moment the issue of Cameron looking ‘frit’ is of relevance only to those who know the political origins of that term – i.e. a handful of people with a healthy (or unhealthy) interest in politics concentrated in London and the odd university town.
I’d put a conservative estimate at 5-10 per cent of the voting electorate.
By contrast, three or more televised debates, backed by advertising on the various channels and almost constant analysis in our national media, would probably draw in an audience closer to 60-70 per cent of likely voters.
When the Campbell rule first emerged, some felt the need for a small addendum – namely if, after two weeks, there was no new angle and nothing to move the story along, there was a small possibility that everyone would end up getting bored and move on to the next enveloping scandal.
Undoubtedly, the broadcast media have the most to lose from the debates not happening and thereby the most to gain from continuing the row, which could prolong the PM’s agony.
But, as the respectful lull in events after the Paris tragedy starts to fade, Number 10 will be devising a fresh story to fill the news vacuum and, they hope, elbow this most Westminster of quarrels off the agenda.
Dylan Sharpe is head of PR for The Sun