War is in the air at Westminster. Battle plans are being finalised, generals recalled and shock troops deployed. In advance of the start of “the long campaign” in January, all parties are transitioning into election mode. Lynton Crosby, the Conservative strategist, has become the pivotal figure at the 4pm Downing Street meeting as David Cameron’s focus shifts from governing to electioneering. Ed Miliband and his office will shortly move from parliament’s Norman Shaw South building to Labour’s Brewer’s Green HQ to merge with the campaign operation. Lib Dem ministers are avoiding the Commons when possible in order to devote their energies to defending their endangered seats.

The Tories and the Lib Dems have spent the period since the Autumn Statement (which Nick Clegg snubbed to campaign in the marginal constituency of St Ives) publicly berating each other. But unlike past ructions over the Alternative Vote campaign, Cameron’s EU “veto”, the constituency boundary changes and childcare ratios, these controlled explosions are designed to benefit both sides politically.

For the Lib Dems, the aim is to demonstrate their independence to the anti-Conservative voters they need to win over to retain the majority of their seats. Of the party’s 56 constituencies, the Tories are in second place in 37. With most of those MPs in Labour-facing areas regarded as “dead men walking”, in the words of one source, it is in these seats that the party’s hopes reside. By talking up the dangers of a future Conservative government, Lib Dems aspire to persuade left-leaning voters that the safest option is to vote for them. For the Tories, the aim is to advertise the defects of another coalition in advance and to convince the electorate that only a majority Cameron administration can be trusted to maintain economic stability, cut taxes, reduce welfare spending and control immigration.

Figures on both sides express the hope that their near-daily disputes will crowd Labour out of the argument, the dynamic that led Alastair Campbell to lament in 2011 that his party was only “the third most interesting”. Although they confront each other as enemies in the marginals of southern England, the Tories acknowledge that they need the Lib Dems to recover if they are to hold off Labour in the battlegrounds of the north and the Midlands.

Some in Miliband’s party see benefits to this new phase of “aggressive differentiation”. By challenging the Tories’ refusal to impose further tax rises on the wealthy and by making the case for borrowing to invest, the Lib Dems lend credibility to policy stances shared by Labour. It is harder for the Conservatives to dismiss Miliband and Ed Balls as deranged Keynesians when their views are echoed by the people they have been in government with for the past four years. The Labour leader’s speech on 11 December, which took aim at the Tories’ own “35 per cent strategy” to shrink the state to its lowest level since the 1930s, demonstrated his confidence in attacking what he regards as a profound strategic error by Cameron and Osborne.

The Lib Dems’ assault on the Tories also exposes them to the charge of inconsistency, a concern articulated by the former minister Jeremy Browne, who urges his party to focus on claiming credit for the coalition’s achievements. When Clegg’s party complains of the “unnecessary pain” planned by the Conservatives, Labour will remind voters that the Lib Dems supported the “bedroom tax”, the tripling of tuition fees and the reorganisation of the NHS. If the Tories are so nasty, why vote for the people who have sat in cabinet with them since 2010?

The greatest irony of the coalition’s internecine warfare is that its members are quietly preparing the ground for a post-election renewal of vows. Both parties are consciously avoiding policy commitments that could prove impossible to maintain in a future negotiation. While railing against the Lib Dems’ proposed “mansion tax”, Osborne has not ruled out introducing a version in the future (having privately supported the option of higher council tax bands in opposition to Cameron). From the other side, Danny Alexander has said of a future VAT rise, “I certainly would not advocate any further increases” – a signal that he fears he may be forced to do so by the Tories. The only official red line drawn by any figure is Cameron’s vow not to lead a government that cannot deliver an in/out EU referendum. While publicly opposed to this policy, Lib Dems privately signal that they would be prepared to accept it in return for concessions such as House of Lords reform and the introduction of proportional representation for local elections.

They also acknowledge that the party’s leadership would rather continue to do business with the Tories than enter government with a Labour Party regarded as irredeemably tribal and impervious to compromise. One MP tells me that the Lib Dems would be prepared to take the path rejected by the Liberal leader Jeremy Thorpe in 1974 and prop up a Conservative government with more votes than Miliband’s party but fewer seats. Cameron – who, as the incumbent, enjoys first preference on any future coalition – can give thanks for that.

No party publicly acknowledges such war-gaming. The Tories and Labour, with good reason, maintain the pretence that they can win majorities even as the polls suggest another hung parliament is likely. No one can be seen to contemplate failure in advance. This stance will be harder to maintain in the heat of a general election campaign when the media and the voters, mindful of the experience of 2010, will demand answers on prospective coalitions. For now, though, the conspiracy of silence endures.