IT WAS, said the Sun with due modesty, "an historic announcement from Britain's No 1 newspaper". And so it proved, as it eclipsed the activities of the parties on their first day of electioneering.
Its front page, proclaiming "The Sun backs Blair", featured on every British television news bulletin on Monday night. It was the main discussion item on BBC2's primary current affairs programme, Newsnight.
Yesterday the Sun's switch to Labour after more titan 20 years of unswerving support for the Tories was the main story in the Financial Times and the tabloid's rival, the Daily Mail. It figured on the front page of every other broadsheet and became the day's hottest topic on radio phone-ins.
If the Pope decided to support Rangers it is unlikely he would get as much media attention as the transfer of the Sun's favour to Labour. Its two-page leading article argued that Tony Blair should be the next prime minister because he is the "breath of fresh air" Britain needs.
"The people", declared the paper, "need a leader with vision, purpose and courage who can inspire them and fire their imaginations. The Sun believes that man is Tony Blair." It wrote off the Tories as "tired, divided and rudderless".
To put this in perspective, we should consider the Sun's electoral record. In 1970, months after Murdoch bought it, the paper gave unstinting support to Harold Wilson's losing Labour Party.
In the two 1974 elections, won by Labour, it remained neutral, with headlines which betrayed a growing cynicism: "The Devil And The Deep Blue Sea" and "May the Best Men Win - And Heaven Help Us If They Don't".
A year later, when Margaret Thatcher became Tory leader, the Sun enthusiastically adopted her and her policies. The paper was her most devoted cheer-leader during her three successive election victories in 1979,1983 and 1987. She always felt the press to be an important ally and believed the Sun responsible for delivering hundreds of thousands of working-class votes from its 10 million readers.
When Thatcher lost the leadership to John Major in 1990, the Sun remained fiercely loyal to the Tories and virulently opposed to Neil Kinnock's Labour Party. In the years up to the 1992 election it continually lampooned Kinnock as a windbag lacking the intellect for high office. That propaganda campaign culminated in a notorious eve-of-election front page which superimposed Kinnock's face on a light bulb, with the headline: "If Kinnock wins today will the last person to leave Britain please turn out the lights?"
Two days later, after Kinnock's defeat, the paper claimed: "It Was The Sun Wot Won It". But this boast lasted only one edition and has been repudiated ever since by its editor and its owner, Mr Rupert Murdoch.
Two major research studies which attempted to discover if there was any truth in the claim came up with entirely different results, while agreeing that about 36 per cent of Sun readers voted Labour. But most media studies academics, journalists and psephologists firmly believe one headline cannot affect how people vote.
They also argue that what a paper says during an election campaign is unlikely to influence voting. Much more important is what the media as a whole say in the years between elections, the drip-drip-drip of prejudice for or against personalities.
In that sense, it would have been inconceivable for the Sun to have suddenly supported Major this time around. Ever since September 1992, when Britain lost billions struggling to remain inside the European Exchange Rate Mechanism, Major has been relentlessly attacked by papers which have traditionally backed the Tories.
The Sun has been a particularly vehement critic, building an image for its readers of Major as a dithering leader presiding over a disunited party.
Throughout this period Sun readers were also registering their disenchantment with the Tories. Apart from a general dissatisfaction with further European integration, domestic concerns about job insecurity and the collapse of the health and education systems engendered dismay.
When Tony Blair assumed the Labour leadership and set about changing the party, and rewriting Clause 4, the Sun tiptoed towards its old enemy. At the same time, Blair's press secretary, Alastair Campbell, set about wooing Tory papers. He has succeeded beyond his best expectations.
A lot has been read into Blair's visit to Australia in the summer of 1995 to address a Murdoch corporation conference. Some commentators are convinced Murdoch and Blair reached an agreement: Murdoch would order his papers to support Blair in return for Blair relaxing rules on cross-media ownership. This would allow Murdoch to take a stake in terrestrial television. It is inconceivable there was any such talk let alone a deal.
The truth is, Murdoch and his senior Sun executives would have preferred to stick with the Tories. Stuart Higgins, the Sun editor, told me on Monday: "The Tories are tired, divided and need a good rest to regroup.
In the paper he wrote: "The Tories have all the right policies but all the wrong faces."
These two quotes illustrate the paper's, and Murdoch's, hidden agenda. Their support for Labour is all about them getting the kind of Tory party they prefer.
In other words, their message to the Tories is: get a new leader, find some new frontbenchers, adopt a rigid policy against the single currency and we'll be back on your side for the millennium.
Meanwhile, echoing Lady Thatcher, the Sun believes Britain is safe in Blair's hands. He has pledged not to raise taxes, not to take privatised utilities back into public ownership and not to repeal anti-trade union laws.
NOT that the paper could do much now to turn back the tide. The latest MORI poll findings suggest 56 per cent of the Sun's readers will vote Labour.
This 20 per cent change from five years ago could have occurred without any guidance at all from the newspaper. You might think readers have been making up their own minds without noticing the daily assaults on Major's character and the growing praise for Blair.
But if readers are not susceptible to what they read in papers over a period of time, what would be the point of companies spending millions on advertising?
No advertiser expects a single "hit" to attract a buyer, relying instead on a long-running series of ads. No editor can change a mind with a single headline. But relentless editorial spin must have an effect. Why not ask Major in six weeks' time?
Roy Greenslade is a media commentator with the Guardian.