In recent weeks, Coulson has made a concerted effort to portray Brown as a ‘ditherer' by inserting the word into many of leader David Cameron's soundbites.
Sources close to Downing Street told PRWeek that the Tory comms chief's methods are increasingly irking the Prime Minister.
A senior source said: ‘Brown is obsessed with Coulson.'
One official is said to have returned from a recent meeting with the Prime Minister declaring: ‘Gordon could not stop talking about Coulson.'
Former News of the World editor Coulson was appointed as the party's communications director in June 2007.
After a shaky start, he is now thought to be the key cog in a relatively well-oiled Tory comms machine, along with strategy director Steve Hilton.
Tory sources said that although he did not come up with the term, Coulson has been instrumental in getting senior Tories to describe Brown as a ‘ditherer' at every opportunity. With many newspapers picking up on the term, Downing Street is now concerned that the damaging tag is starting to stick.
Downing Street also believes that Coulson has been fuelling rumours that Brown is depressed and on the edge of resignation.
Coulson also had a hand in undermining Brown's recent three-day trip to the US. The Prime Minister was forced to break off from talks in Washington to persuade junior minister Angela Smith not to quit over the abolition of the 10p tax rate after Coulson managed to encourage shadow cabinet ministers to flood the airwaves with the message that ‘this government is unravelling by the hour'.
In the Tory press office, Coulson is backed by two heavy hitters with significant tabloid experience: head of press Henry Macrory is a former Daily Star political editor, while veteran news manager John Deans spent years at the Daily Mail.
Brown's top media handlers are former Treasury aides Damian McBride and Mike Ellam. Former Brunswick CEO Stephen Carter oversees comms as Brown's chief of strategy.