After every high is the inevitable comedown. The Lib Dems have slumped to 15 per cent in the latest polls. So much for political honeymoons. But let's not be under any illusions. To form a coalition is a messy and arduous affair, made more difficult still by a tribal political system that favours single-party (over multiparty) governments. For two parties to come together and compromise on a range of issues, after a hard-fought election campaign in which their leaders berated and mocked one another, was a near-impossible task. It was inevitable that there would be cries of betrayal and a backlash from supporters on both sides.

Yet the Tories' poll ratings are up, while the Lib Dems' are down. I suspect that this is because the public holds the latter to higher standards. The third party, after all, positioned itself as a fresh, honest and decent alternative for voters disillusioned by the cynical and negative triangulations of the two main parties. The Lib Dems were the insurgents promising reform and renewal. To vote for Nick Clegg and Vince Cable was to vote for a "new politics", or so they wanted us to believe. Yet, after only two months in office, it has become obvious that the Liberal Democrats are as disingenuous and as ruthless as their rivals.

On-message

Take the issue of public spending cuts to reduce the Budget deficit. The Lib Dems campaigned against, in the words of Cable, "cutting too soon" and too fast, as well as the Tory plans for "job cuts". They have since U-turned on both the timing and the scale of the austerity measures; Cable claims he was persuaded of the case for deep and early cuts, "not by other politicians, but by talking to the most senior officials in the government and the central bank [who said] that we had to act".

Really? Why then had the Lib Dem negotiators already acceded to a "significantly accelerated" deficit reduction plan involving "cuts of £6bn . . . within the financial year 2010-2011" in the coalition agreement document that they signed with the Tories, prior to a single member of their party stepping foot inside a government department or having a conversation with Mervyn King, the governor of the Bank of England ? Why did they urge Labour, in parallel coalition negotiations, to make cuts sooner than planned? "I was incredulous when David Laws and Danny Alexander made it clear that they wanted substantial cuts this year," the shadow education secretary, Ed Balls, told me. "And Chris [Huhne] was supporting them." Balls added: "I said it would be a breach of both our manifestos, but they didn't seem too bothered about going against their own manifesto."

Then there is the issue of the VAT increase to 20 per cent. "Unavoidable," squealed Alexander, the Lib Dems' placeman at the Treasury, echoing the Tory line. But he knows that the regressive VAT hike was entirely avoidable. Whatever happened, for example, to the mansion tax on properties worth more than £2m or restricting tax relief on pensions to the basic rate of income tax - both commitments included in the Liberal Democrat manifesto?

A recent YouGov survey showed that 48 per cent of those who voted Lib Dem were less inclined to do so again as a direct result of the rise in VAT. The party leadership could have held its hands up and apologised for running pre-election posters warning of a "Tory VAT bombshell" - which the Lib Dems have since helped detonate. Instead, Clegg chose cynically to pretend that his party had always backed the principle behind such a move. "When it comes to a choice between taxing what people choose to buy and taxing work," wrote the Deputy Prime Minister in the Independent on Sunday on 27 June, "it is liberal to come down on the side of consumption rather than payroll taxes. It has been part of a long liberal tradition, from John Stuart Mill to Jo Grimond . . ."

Hmm. Does the Lib Dem leader need to be reminded of Lloyd George and his People's Budget of 1909, which introduced a "super-tax" on the highest earners? "Clegg is rewriting history to suit himself," says the political historian and SDP founder member David Marquand. "By the early 20th century, radical and progressive Liberals did not just support direct taxation; they deliberately increased direct taxation on the rich so as to pay for social reforms."

But not any longer. As the vice-chair of the Liberal Democrats' Federal Policy Committee Richard Grayson explains on page 30: "While many Liberal Democrats see the coalition as a creature of circumstance, its ideological basis lies in a strain of centre-right, small-state liberalism in the leadership of the Liberal Democrats." And it is a leadership intent on making this centre-right coalition work, no matter what the future electoral cost will be to the party itself. Dissent is slowly being marginalised and, in the words of a source on the left of the party: "The inner Clegg circle is now in the 'control the message', top-down phase of the early New Labour years."

It is said that, in private, Clegg's allies have described Lib Dem MPs allied to the Social Liberal Forum - the centre-left, internal party pressure group that has been critical of the Budget - as "idiots". Meanwhile, those Lib Dem backbenchers sceptical of the coalition's plans to introduce Swedish-style "free schools" are being frogmarched by the party's high command into the Department of Education. They emerge, according to one source, "starry-eyed from meetings with Michael Gove. They've gone in wanting to hate him but come out in awe of him."

Dark arts

For years, the Liberal Democrats have piously presented themselves at the national level as whiter than white, while engaging in ruthless, negative and often dirty campaigns at the local level. But no more. Nationally, the party in government has shown itself to be adept at the manipulative and dark arts of British politics.

“This is more like the old politics than the new," says a disgruntled Lib Dem source who believes that Clegg, like Tony Blair in his pomp, will strike a confrontational pose at the party's annual conference in Liverpool and face down activists unhappy with the Budget and his political marriage with the Conservatives.

Back in 2001, addressing the Labour party conference in Brighton, Blair rejected the accusation that the New Labour project had simply been an election-winning ploy: "It's worse than you think. I really do believe in it." It's a line that Clegg might consider borrowing for his own conference speech this year.