But steadily it came to have the opposite effect because it looked so obviously inauthentic and calculating. Before the Iraq War when Tony Blair really did discover his irreducible core — muscular liberal interventionism — he was either not believed by significant numbers of Britons or suspected of having questionable motives. They could see the wiring and diagnosed fakery.

The parties have only just begun to grapple with the consequences of these developments. But it is worth reminding ourselves why they got themselves so hooked on imagery and attempts at media manipulation in the first place. Watching The War Room again it is not hard to see why Clinton had such a dramatic impact on Labour in the 1990s. Here was a training film for progressives, frustrated by the seemingly endless dominance of their conservative opponents. In Britain, among the Labour über-modernisers who were deeply depressed by yet another general election defeat in 1992, the effect of the Clinton victory and the ruthless way it had been secured was electrifying.

Labour had made earlier doomed attempts at improving its marketing, under Neil Kinnock's communications chief Peter Mandelson. But those efforts looked juvenile when compared to the glamorous game of hard-ball played by Clinton's campaign professionals. Tony Blair and Gordon Brown visited Washington in 1993 and were given a warm welcome by a new administration which was keen to tutor potential allies. John Major's Conservatives had aided the Republicans in the 1992 election and Clinton would not forget it.

Awe-struck Blair and Brown concluded that Labour needed to do what Clinton, Carville and Stephanopoulos had done. But back in London, the Labour leader John Smith was unreceptive and sceptical. Smith was an old-fashioned leader, who thought that it was best to start by applying your principles and beliefs to the problems of the day. Then, in as unflashy a way as possible, you should make the case and hope that sufficient numbers of Britons would vote for your programme and party.

The modernisers thought that this had been precisely the problem in the 1992 general election, when Smith's tax-raising shadow Budget cost Labour so many votes. Far better, surely, to work out what voters wanted to hear and to let them hear it. Principle was not being dumped entirely, they said, it was just being updated with historic weaknesses ironed out and potential negatives abandoned. Later, this would cause many problems when the party tried to say one thing on tax but do another.

When Smith died suddenly in 1994, Blair and Brown had their chance and set about implementing what they had learnt in Washington. The tough tabloid hack Alastair Campbell was hired as communications chief. The late Philip Gould, the pollster who had done so much to help Kinnock, became even more of a crutch for Blair. For the 1997 election Labour built a state-of-the-art campaign headquarters in Millbank Tower in Westminster and called it (surprise, surprise) the War Room, consciously invoking the spirit of Clinton's election and subsequent re-election in 1996.