Omnicom's GMMB was one of two advertising agencies working for Kerry's campaign. The other is political media consulting firm Shrum Devine and Donilon, which remains on board with the Kerry campaign.
The two agencies had been working together under a partnership dubbed Riverfront Media, which is now effectively over.
The Kerry camp has said it will add further creative resources later in the campaign. GMMB will not part company completely with Kerry -- it will continue to handle media planning and buying for the senator.
In a statement Jim Margolis, a partner at GMMB, said: "Unquestionably being a part of Senator Kerry's Presidential campaign has been one of the most important efforts our firm has ever undertaken. However, due to proposed contract changes with the media firms, we could not come to an understanding that we regarded as workable."
The parting of ways with its creative agency comes as figures reveal that President Bush has poured about $40m (拢21.9m) into television and radio advertising with $110m to hand, outspending the Democrats by a factor of two to one.
Much of Bush's radio and TV campaign has taken the form of negative advertising attacking the record of his rival Kerry, who is estimated to have spent about $6m on TV ads, with independent liberal groups spending a further $14m.
The power of the Bush campaign has so far, according to US reports, buried Kerry in some media markets since March 4, when the President started running ads on local TV station in 18 states, on national cable networks and on radio stations.
It is unclear how much effect Bush's re-election campaign is having. He recently came under fire and was forced to defend television ads that used images of the wreckage of the World Trade Center.
The use of the images in two of Bush's first television commercials for his Presidential campaign has prompted anger from the families of those who died in the 9/11 attacks.
In one of the ads, "tested", footage of the wreckage of the World Trade Center is shown along with that of firefighters who worked on the scene. In a second, "safer, stronger", the same footage is used with the words "a day of tragedy" superimposed.
Independent polls show Bush and Kerry remain in an almost dead heat, with Bush clawing back some ground from early March when Kerry, fresh from his primary wins, led Bush, who had yet to begin his campaign.
US reports say that the Democratic candidate's own polling shows Kerry has dropped five to six percentage points in key states since Bush began his advertising campaign.
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