Too much informality, hostility to the media and too much concentration on the structures of government rather than on policy could prove disastrous, George Stephanopoulos said in a speech in London.
Mr Stephanopoulos, who advised new Labour on its campaign, returned to London this week to see his friends in the party settling into government.
The party had had a "nearly perfect" first week, he told an Institute of Public Relations awards ceremony, but there were danger signs.
The American President's decision to be photographed jogging in his first days after election had proved damaging later when he needed to present a statesman-like air, he said.
"When there is a crisis the public are going to want someone in that office who is a little bit stronger and a little bit more mysterious than they are.
"What we need is a little bit less 'call me Tony' and a little bit more 'Yes, Prime Minister'."
Mr Stephanopoulos, a former director of communications in the White House, also warned that he had done "this really stupid thing" in closing his door to the press.
"I think there's a little bit of this tendency within Labour, to think they won this election in spite of the press," he said. The key thing for Labour would be to focus on the substance of its policies.
Allowing the media to write endlessly about centralisation and party discipline would give people inside the government "the perfect way to take down their leaders" he suggested.
"They have done everything right while we did everything wrong in the first week. I know how quickly the promise of a new start can go sour," he said.
Mr Stephanopoulos was receiving an award for his own public relations achievements from this year's president of the institute, Simon Lewis.
Seeing the "exhausted satisfaction" in his Labour friends' faces as they discovered their new offices and negotiated the corridors of power brought to mind the confused, happy expressions of Dustin Hoffman and Katharine Ross in the final scene of the film The Graduate, he said.
"All you think is that they don't have any idea what they're doing.
"Labour's actions in the first week have proved that is very wrong," he said.
"Their first week has been nearly perfect and as someone who had a nearly disastrous first week in the White House I look at it with special admiration," he added.