When Nick Clegg was first elected Liberal Democrat leader David Cameron asked him and his wife over for dinner at his home in Notting Hill. There seemed to be a natural affinity between the two youthful modernisers — both have public school charm, young children, high-flying spouses and centrist views. But there was hardly time for Samantha to order the lamb from Lidgate’s before the invitation was turned down. The Liberal Democrats have been resisting the advances of the liberal Conservatives ever since.

Paddy Ashdown used to say that during his meetings with Tony Blair he had to press a metaphorical nail into his palm to resist the charm. Now it is the Tories who are love-bombing the Lib Dems.

In an article at the weekend, Mr Cameron claimed there was “barely a cigarette paper” between him and Mr Clegg on civil liberties, the environment and education. Yesterday Eric Pickles, the Conservative chairman, sent out an e-mail urging Lib Dem voters to “lend” their support to the Tories and form a “progressive alliance” against Labour at the next election. “The Conservative Party is the home of liberal democracy,” he said.

Whether marching arm in arm with Joanna Lumley over the gurkhas, campaigning against ID cards or opposing the third runway at Heathrow, Mr Cameron wants to be seen as Mr Clegg’s new best friend. It’s not just about sucking up, of course, it’s also about hoovering up support. Vote Blue Go Green the Conservatives like to say — and it takes yellow to turn blue green.

The nicer Mr Cameron is to Mr Clegg the nastier Mr Clegg becomes about Mr Cameron. On Sunday he called his Tory opponent the “conman of British politics”. Vince Cable claimed that the Conservatives would put up VAT to 25 per cent. The Lib Dems have set up a “Tory attack unit” that will concentrate on highlighting George Osborne’s inexperience.

Mr Clegg says he wants to replace Labour as the “dominant force of progressive politics”. But politically he looks increasingly confused. One minute he outflanks the Tories to the right by advocating “savage” public spending cuts, the next moment he outflanks Labour to the left by proposing a tax on homes worth more than £1 million. The coalition he holds together is under strain. No sooner had Mr Clegg suggested that child benefit might be means tested than Steve Webb, the party’s work and pensions spokesman, declared that there should be no change. “Leaders of the Liberal Democrats don’t always get their way,” said another MP, Evan Harris, on the subject of student tuition fees.

The truth is that the Liberal Democrats were knocked off course by the election of a socially liberal Conservative leader and they have still not recovered their balance. As Labour lurches from one disaster to another, and public opinion hardens against Gordon Brown, this should, as Mr Clegg says, be “the Liberal moment”. But it isn’t. In the recent European elections, the Lib Dems won just 14 per cent of the vote. Too large to be political outsiders and too small to be serious contenders, the party is trapped in its own ambiguities.

When the MPs’ expenses row broke, the Lib Dems were treated as part of the Westminster elite, so the protest votes went to fringe parties instead. Although they made the right calls on the economy, they do not get much credit because their brand is unclear. They have lost the policies with which they were identified at the last election — opposition to the Iraq war and a pledge to scrap tuition fees — and replaced them with a series of mixed messages. Mr Cable is more trusted by voters than his rivals, but because he is perceived to be above politics, not because he is a Lib Dem.

There is a fundamental flaw in the Liberal Democrat approach. They behave as if they are the same as Labour and the Conservatives when, as the third party, they are in a different position. There are “Nick Clegg for Prime Minister” mugs on sale in Bournemouth but nobody seriously believes that he is going to be moving into No 10 next year. Privately, everyone in the party knows it. One frontbencher recalls having a furious policy row with a colleague — it had been heartfelt and bitter but also, he later realised, absurd, as neither of them was ever going to be in a position to implement their plans.

Publicly, though, the Liberal Democrats try to pretend they are a potential government rather than acknowledging that the way in which they are most likely to have real influence is by holding the balance of power. They set themselves up as the party of honesty, who will tell the truth about fiscal restraint, but on the issue over which they have most control — the role they would play in a hung Parliament — they offer only obfuscation. They define themselves constantly in terms of the other two parties, then when it comes to the crunch they refuse to say what demands they would make in return for their co-operation. The C-word Mr Clegg will not use is coalition.

Of course there is a difficult line to tread: the Liberal Democrats can’t just look like they are rolling over in front of the bigger parties. But, as one MP puts it, being a kingmaker is almost as good as being the king. Few in his party think that Mr Clegg would prop up Gordon Brown if Labour had lost the popular vote, so he should make his intentions clear. He would look more credible if, instead of calling Mr Cameron names, he set out how he would be a liberal restraining influence on a Tory government.

He might also benefit from candour. If the Lib Dems could turn the election into a poll about what they would bring to a coalition, they would maximise their support because a vote for them would no longer be seen as a wasted vote. Paradoxically, they may do best by admitting their limits. As sports stars like to say: “You can’t win unless you learn how to lose.”

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