If the coalition needed romance to survive it would have ended long ago.

It is almost impossible to avoid comparing the coalition to a marriage. It was launched in a rose-fringed garden in springtime by two men whose sudden discovery of political affinity looked uncannily like romance. David Cameron joked about his relationship with Nick Clegg as a civil partnership.

The reality quickly became more like a joint business venture, with a small number of executives - the ministerial "quad" of Cameron, Clegg, George Osborne and Danny Alexander - taking the key decisions. Their respective parties have played the role of restive shareholders.

In recent weeks, an eruption of political turbulence has raised questions over the viability of the whole project. Preparations for the budget have been played out in a noisy parade of competing tax initiatives. Lib Dem ministers have flaunted their instinct to raid the rich, cast as mansion-dwellers and tycoons, for a bigger contribution; Tory backbenchers have restated their impulse to cut some fiscal slack to the same people, portrayed instead as noble wealth-creators.

Meanwhile, the Lib Dems have tied themselves in knots over NHS reforms. Delegates at the party's spring conference voted to express gladness that the government's health bill has been mangled by amendment into something that thwart Tory plans, while refusing to approve of what it has become instead.

Budget pantomime

These difficulties expose a flaw in the coalition business model - the lack of clarity about what the enterprise is actually selling. From the Conservative perspective, the Lib Dems were recruited to help with the dirty work of deficit reduction. The junior partner signed up, partly to share any credit for restoring order to the public finances, but also in the hope of persuading the country that having two parties in government might be as effective as one - or even more so. Cameron needed Clegg to present austerity as a national project; Clegg needed Cameron to promote coalition as part of Britain's political repertoire. It felt like the same thing but wasn't.

Now both offers look flimsy. From the budget pantomimes, the public might conclude that coalition does not facilitate the smooth implementation of austerity, since the two partners disagree over who should bear the heaviest burden. The row over the NHS showcases a failure of two-party government to achieve happy compromise, producing unloved policy monsters instead.

Things are not set to get any easier. The coalition is entering a phase that is tricky for any administration. Ambitious promises have been made but change is not yet visible on the ground. There is a lot of unglamorous implementation work to be done, a task that will, according to the muttering of Downing Street aides, test the limited managerial capabilities of several ministers. This "delivery" period also bores much of the media and leaves backbench MPs underemployed. Under such conditions, mischief thrives.

Unhelpfully, the next big legislative odyssey is House of Lords reform, which is a project perfectly designed to aggravate coalition tensions. Clegg's plan to have 80 per cent of the upper chamber elected is resisted by the Lords themselves (not surprisingly, since it would make most of them redundant). It is also loathed by Tory MPs, many of whom will defy the whip if ordered to vote in favour.

Many Lib Dems suspect that such sabotage would be discreetly sanctioned by No 10. Lords reform might offer Cameron a policy area where backbenchers can take out their anti-Clegg frustrations without affecting vital economic or social policy. It could be, as one senior adviser puts it, "the bit of the garden where it's safe to let them off the leash". It would not be the first time a Lib Dem constitutional change ended up as a chew-toy for the Tory right. The sanctioning by Cameron and Osborne of attacks on Clegg's integrity in the Tories' referendum campaign against electoral reform last year has not been forgotten. In the event that the Lords bill is destroyed, vengeance could be enacted by blocking constituency boundary changes - plans that the Tories hope will chip away at a perceived pro-Labour bias in the way parliamentary seats are drawn on the map.

None of this will be edifying and most of Westminster is mystified as to why Clegg should embark on such a hazardous journey to achieve something for which there is no public clamour. Lords reform was so far from the Lib Dem leader's mind last year that reference to it was accidentally omitted from early drafts of his annual party conference speech and reinstated belatedly as a four-word subclause.

Those closest to the Deputy Prime Minister say his sudden attachment to the idea comes from a romantic urge to leave some constitutional imprint on history; a chapter in the annals of reform. "One for the grandchildren," is how a source describes it. This exalted sense of Liberal destiny is far removed from the perspective of those rank-and-file Lib Dems who must defend the party's governing record in council elections in May.

Relying on recovery

It is a problem with which grass-roots Tories can identify. Cameron is much more popular than Clegg but he has consistently failed to articulate a governing mission that resonates with voters at kitchen table level. The "big society" was about as practical in that respect as Lords reform. Downing Street strategists are painfully aware that their election prospects rely too heavily on hopes of an economic recovery, national stoicism in the face of austerity and the whiff of implausibility that hangs around Ed Miliband.

It is easy enough to map out the ways in which relations between Lib Dems and Tories will deteriorate. The trajectory is towards legislative deadlock and debilitating personal acrimony. That doesn't mean the project will break apart any time soon. Cameron still needs the Lib Dems in government so he can look like a man getting to grips with a national economic emergency; Clegg still needs the Tories to show that multiparty government is workable. If their partnership needed romance to survive it would have ended long ago. But this is business and coalition is the only product they know how to sell.