“NO PERMANENT friends or enemies, only permanent interests”, goes a well-worn axiom of realist statesmanship. The formation in 2010 of Britain’s first coalition government since the second world war showed that the prospect of power could trump large ideological differences between two parties. But diplomatic relations between David Cameron’s Conservative Party and Nick Clegg’s Liberal Democrats have reached the table-thumping stage.

Last year saw bitter arguments over House of Lords reform and constituency boundaries. It ended with the spectacle of Mr Clegg theatrically rolling his eyes during the autumn statement, in which George Osborne, the Tory chancellor, announced new tax and spending measures. This year is likely to bring rows over press regulation, Europe, welfare cuts, green energy and departmental spending. Such is the antipathy that plans for a revised government programme were first delayed, then shelved, for fear of uncontainable hostility between the two sides. Instead, a more modest package of new measures will be announced in the spring.

Astonishingly, the 2015 election already looms over the coalition. Conservative thinkers are focused less on improving this government than on winning a workable majority next time. And the governing parties are preparing for combat. The Conservatives are targeting Liberal Democrat seats, convinced that their partners’ terrible poll figures will not recover. The Lib Dems will concentrate on fighting off Conservative challengers: at least they do not have to defend the coalition in such races, as they do when fighting Labour.

The process of “differentiation”, in which the parties distinguish themselves from the coalition, has got under way early. Lib Dems miss no opportunity to remind voters that, unlike their Conservative colleagues, they support a “mansion tax” on expensive homes. An internal memo leaked in December revealed that the party plans to present itself as the conscience of the coalition, restraining Tories from “looking after the super rich while ignoring the needs of normal people”. The Conservatives, in turn, blame the sluggish economy on Lib Dem opposition to deregulation and to further spending cuts.

The launch of the coalition was accompanied by ambitious talk of a grand realignment of Britain’s political centre. Mr Cameron hailed “not just a new government, but a new politics”, one in which “the national interest is more important than the party interest” and “where co-operation wins out over confrontation”. The partners declared the coalition agreement “more radical and more comprehensive” than their own electoral manifestos. The coalition, some suggested, was not a compromise, but an ideal. This brave new politics has given way to a cold war between the two sides.

But it has not disappeared entirely. Bickering in public and on the back benches belies a strikingly businesslike atmosphere in Whitehall. Conflicts are carefully controlled: last year Mr Clegg even discussed his mutiny on boundary change with Mr Cameron before announcing it. In a sense, coalition discord is the opposite of the internal feud that preoccupied the last Labour government. Supporters of Tony Blair and Gordon Brown fought much harder behind the scenes than in public.

Decision-making at the top of the coalition is a case study in the realistic and mature management of discord. Senior ministers seek consensus in a series of conclaves. The most significant is the “quad”, made up of the prime minister, the deputy prime minister, the chancellor and the chief secretary to the Treasury. Here, at Monday morning parleys between the two party leaders and in meetings of Cabinet Office panjandrums, compromises are brokered, assurances given and caveats established. The need for both parties to approve new measures has revived cabinet government: debate is more thorough and better documented than before.

Jaw-jaw, not war-war

It helps that neither party wants an election. Both are doing poorly in the polls, and advisers caution that the public punishes leaders who fail to work together in the national interest, particularly at a moment of economic peril. The dangers of the nuclear option—one of the partners pulling out of the coalition, triggering a Conservative minority government followed by a new election—are a robust deterrent.

But as a small, intricate network of senior figures forges consensus, a serious rift is growing between leaders and their parties. Intense dealmaking at the top of government makes it harder to modify measures once agreement has been reached. Tory MPs, many of whom are already suspicious of leaders who share the Cleggites’ metropolitan, liberal outlook, feel shut out of the policy-making process. One backbencher criticises decisions “handed down from on high” by the “four or six” people who hold the reins of power. Grassroots party members are intensely grumpy at the concessions that coalition demands. There are rumblings among the Lib Dems, too, though of a different order. Mr Clegg, who is deeply unpopular and tarnished by his proximity to the Tories, looks increasingly vulnerable to a leadership challenge from the left of his party before the next general election.

Mr Clegg and Mr Cameron are increasingly pulled in two directions. The public expects them to get things done, which requires flexibility and guile. Yet they are also expected (particularly by MPs and activists) to display solid principles and fierce loyalty to their tribe. The coalition partners are torn between nurturing their working relationship and managing their parties.

In the year ahead, the two leaders will attempt to reconcile these two priorities. There may be more controlled explosions like Mr Clegg’s rejection of boundary change last summer. Differences of opinion once kept behind closed doors will probably be aired more openly. Both sides will talk up the fruits of their realpolitik, too: the coalition has been poor at selling its achievements. If they get the balance right, it will serve both parties’ permanent interests. If they get it wrong, 2013 may be the year the coalition cold war heats up.