We should be sceptical about the notion of radical shifts in mood in politics. James Callaghan said at the time of the 1979 election: "There are times, perhaps once every 30 years, when there is a sea change in politics. It then does not matter what you say or what you do. There is a shift in what the public wants and what it approves of. I suspect there is now such a sea change and it is for Mrs Thatcher."
The truth is that the old pro was covering up for his failure to hold an election in the autumn of the previous year, which he might well have won. Sometimes, after the event, elections can be seen as important turning points in the story of the nation, but it is the essence of democracy that such outcomes are not preordained. Just as it is possible that Callaghan could have averted defeat, so Gordon Brown – or, if not him, another Labour leader – could still win the next election.
Much of this depends on Mr Brown, who has shown steadiness under fire in recent weeks, and on his party. But a great deal depends on David Cameron, and how he uses the momentum generated by the Conservative Party's best showing in local elections since a man landed on the Moon. Mr Cameron is entitled to luxuriate for a moment in the embarrassment of those who have been telling him that he "should be doing much better". Just as Boris Johnson is entitled to take some satisfaction from contradicting those who once derided his attempt to become Mayor of London as a joke.
On Thursday, the combination of outstanding results in a patchwork of local councils around England and Wales and victory in London, where 2.5 million people voted, proved the electability of the new Conservative brand.
There is a cloud to Mr Cameron's silver lining, to which we will come in a moment. First, we should remark on Mr Johnson's response to his election. The generosity of his tribute to, and promise to work with, Ken Livingstone and Brian Paddick was statesmanlike. So was his recognition of the "many whose pencils hovered for an instant before putting an X in my box".
This was matched by the tone of Mr Livingstone's farewell, which contrasted sharply with his aggression towards some parts of the media.
There are dangers to Mr Cameron in Mr Johnson's election, however, as we report today. The most important is not Boris's buffoonish act or his poshness, but his instinctive opposition to key elements of Tory modernisation – the very elements that make Mr Cameron a credible contender for national office after the Tories' long years in opposition.
Above all, Mr Johnson's green credentials are suspect. Although his father, Stanley (who would like to replace him as MP for Henley), was a pioneer of environmentalism, Boris has been until recently a climate-change sceptic. Seven years ago, he called George Bush's rejection of the Kyoto protocol "right not just for America but for the world".
After Mr Cameron went green, Boris said he was "terrified to dissent from the growing world creed of global warming", but that his mind was still "bubbling with blasphemous thoughts". So when he said in his manifesto for London that "City Hall should... strongly support efforts to tackle climate change", we are entitled to doubt the sincerity of his conversion. Especially as one of his pledges is to ditch Mr Livingstone's planned £25 congestion charge on larger vehicles.
Now is the worst time for the Conservative Party to retreat from green politics. Over the past year, Mr Cameron has lost momentum on the subject. His early boldness in calling for binding annual limits on greenhouse gas output, for example, has not been followed through, which might have prompted the likes of Mr Johnson to think that a bit of trendy green posturing was simply a device to reposition the Tory brand.
That leaves an obvious gap in the political market for any attempt by Labour to recover lost ground. The Independent on Sunday has chided Gordon Brown as a laggard on the environment, but the the Prime Minister is catching up, and some progress on issues such as renewable energy is being made.
We continue to believe that Mr Cameron's understanding of the green imperative is genuine, if tempered by a view of the politically possible, whereas that of Mr Johnson is mostly notional. We are left hoping that Boris will not be a strong independent voice for London but will be kept on a tight leash by Mr Cameron.
As the most important elected Tory politician in the land, Mr Johnson can be an exemplar of the new Conservatism, or he can use City Hall as a platform for satirical opposition to Mr Brown. In his first hours he has struck the right note, but it will be by his actions over the next two years that he – and his party – will be judged.