"Cleggalomaniac." "Vote for Clegg is vote for Brussels." "Clegg in Nazi slur on Britain." "Clegg, the donors and payments to his bank account." The Sun and Daily Mail don't do things by halves. When the message goes out that a leading politician, hitherto ignored, poses a mortal threat to the nation, it must be hammered home, not once, but page after page, headline after headline, day after day; not just on news pages, but in every available opinion column; not only in words, but in pictures. One can only stand, admire and get out of the way, as one does when herds of animals stampede in unison across the African savannah.

Since Nick Clegg was declared the "winner" of the first televised election debate, no shred of evidence to discredit him has been left unused. Eight-year-old newspaper articles, expenses claims we were told about months ago, internal Liberal Democrat memos that long preceded his becoming an MP - let alone party leader, have been unearthed by Fleet Street's fearless sleuths. It wasn't just his pro-Europe views, his dodgy finances or his cosmopolitan ancestry. He attended Westminster School when the place was awash with drugs. He worked for a lobbying firm that once represented Colonel Gaddafi and Vladimir Putin. The papers failed to find evidence that Clegg shook hands with a man who danced with Myra Hindley but not, I am sure, for want of trying.

Out of the loop

What explains the hysteria of these attacks, which far exceed anything directed at Gordon Brown? There is no doubt political correspondents for right-wing papers were briefed and given "angles" by Tory spin doctors. But to see it all as a Tory plot is too simplistic. Parties routinely brief friendly journalists on campaign themes. They do not always get such an enthusiastic, single-minded response.

Newspapers do not like the idea of a hung parliament or of the Lib Dems achieving anything like equality with the other parties. Football matches do not have three sides. A three-sided contest flouts the elementary rules of drama and narrative. At elections, several million people vote for the Lib Dems, but no newspaper consistently backs them. Journalists do not cultivate Lib Dem contacts, read their policy papers or, in the Sun's case, even attend their conferences (as an ex-editor of that paper revealed recently). The prospect of Lib Dems having crucial influence on the next government, and even becoming ministers, leaves newspapers feeling out of the loop. They therefore warn of national ruin if there is a hung parliament and/or coalition government.

But for editors and proprietors, deeply personal issues are also involved. The Mail, for example, has always been and remains a Tory paper. But its editor, Paul Dacre, does not care for David Cameron: he is too posh, too slick, too modish, too liberal on social issues, too much like Tony Blair. Gordon Brown is none of those things; he shares with Dacre (or Dacre believes he shares) a stern moral compass. So the Mail, while going hard for Labour, went easy on Brown, faintly praising him on occasions. This made its initial election coverage strangely muted. Clegg's success in the TV debate was a liberation: at last, the Mail had a clear enemy - as posh as Cameron, and even more like Blair - and it could proceed in its finest traditions of character assassination.

Then there is the Sun, a paper that derives much of its identity and authority from its reputation for backing winners. Its claim to be the paper "wot won it" for John Major in 1992 is central to its brand image. Its proprietor, Rupert Murdoch, wishes to exercise power, not so much to influence policy or even to secure business advantage (though he uses his closeness to governments to both ends), but because he likes power in the way other people like snow-capped mountains or Greene King real ale. Murdoch does not despise Brown, as he ­despised Neil Kinnock. Like Dacre, he has a soft spot for the PM and, as a populist, does not warm to Cameron. But last autumn, Murdoch was persuaded by his son James, his representative in London, and by Rebekah Brooks, News International's chief executive, to abandon New Labour and embrace Cameron.

Murdoch Sr made the wrong call in the US presidential elections, backing John McCain rather than Barack Obama. Now he fears ending on the wrong side in the British election, too. Politicians on both sides of the Atlantic would then know they can survive without Murdoch. They can ignore his calls and his demands for an audience. To a man so accustomed to access, being frozen out would be unbearable.

The setting Sun

No wonder that (according to a blog by Murdoch's biographer Michael Wolff) Tom Newton Dunn, the paper's Old Etonian political ­editor, is telling colleagues: "It is my job to see that Cameron fucking well gets into Downing Street." No wonder Brooks and Murdoch Jr burst into the Independent's offices to berate the acting editor, Simon Kelner, for his paper's advertising slogan: "Rupert Murdoch will not decide the outcome of the election. You will."

The longer Clegg rides high in the polls, the worse for the Sun and other papers. They have "exposed" the Lib Dem leader as a liar, a fraud,

a hypocrite and a foreigner. They have declared Cameron the clear winner of the second televised debate. "The Cam Back Kid", the Sun announced in one of its more excruciating puns. But as I write, the polls still predict a hung parliament and most suggest the Lib Dems will get around 30 per cent of the vote, perhaps more.

The editors and proprietors have pulled the usual levers. They should, by now, have pulverised the opposition. Nothing has happened, and on Twitter the people mock. An awful truth is dawning: even newspaper readers no longer take newspapers seriously.

Peter Wilby was editor of the New Statesman from 1998-2005.