Despite their public hostilities, Ed Miliband sent Nick Clegg a private message recently, urging him to prepare for the day when the Liberal Democrats break with the Coalition, and hinting that Labour would do business with them if they did.
The verbal message, passed through an intermediary, is fascinating at several levels. The Labour leader is implementing his advertised strategy of wooing the Lib Dems as well as hoovering up their supporters. It's a nod and wink that he might be able to work with Mr Clegg, despite suggesting during Labour's leadership election he would demand his head in any future Lib-Lab arrangement. Also interesting is the Lib Dems' reaction to Mr Miliband's tentative olive branch. If anything, Mr Clegg is getting in deeper and deeper with the Tories rather than thinking about breaking up. That doesn't mean a permanent Coalition or a merger of its two parties. Their relationship is a marriage of convenience for both. It allows David Cameron to complete the detoxification of the Tory brand. But it also allows Mr Clegg to do something very similar for his party. Its private polling shows that the main reasons people are reluctant to vote Lib Dem is that they regard a coalition and hung parliament as a bad thing; and doubt the party's economic credibility and ability to run anything. "If this works, we'll be halfway there," one Clegg ally explained. In other words, the party would be taken seriously as a contender for power under its own steam.
This explains the Clegg strategy. From the outside, it can look as though he is being steamrollered by the Tories as he swallows deep public spending cuts and makes a spectacular U-turn on university tuition fees. But even if it means taking a hit in the opinion polls there is method in the apparent madness. Mr Clegg's calculation is crude but probably correct. If the Government's deficit-reduction strategy is eventually perceived by the public to have worked, the Lib Dems may get some credit – but only if their hands are "dipped in blood" now. So next week he will stand shoulder to shoulder with Iain Duncan Smith, the Work and Pensions Secretary, for the unveiling of welfare reforms, and will defend cuts in housing benefit which some Lib Dem MPs find hard to stomach. If the strategy fails, then both parties would go down with the ship anyway; the Lib Dems won't get any credit for half-heartedly supporting the cuts after keeping the Tories in power.
Instead, the Coalition partners use each other skilfully as political cover. The Tories don't look so nasty when they make cuts because those nice Lib Dems are backing them. The two parties also think hard about who announces what. If Mr Clegg had trumpeted this week's decision to allow most prisoners the vote, it would have caused Mr Cameron even more problems with Tory MPs and supporters. So the Tories took the reins, emphasising the Prime Minister's reluctance but billing the decision as a way of avoiding a big compensation bill following a ruling in the European Court of Human Rights. Similarly, the Liberal Democrats are confident of softening the Tories' stance on immigration but won't shout it from the rooftops. Mr Clegg won a highly significant victory when the Government delayed a decision on whether to renew the Trident nuclear missile system until 2015 but didn't open the champagne in public. It suits Tories and Lib Dems to be all in it together and the result is a remarkably competent and disciplined government. Both Mr Cameron and Mr Clegg know there will come a time when the parties eye the next election and become more tribal. Trident is a good example of where they will diverge.
For now, though, things can only get closer. Tory and Lib Dem minds are turning to the second half of the four years of full-blown Coalition. Oliver Letwin, the Tory policy guru whose title of Cabinet Office Minister belies his huge influence, and Danny Alexander, the Lib Dem Treasury Chief Secretary, will soon start work on a "second-half" strategy.
Next week, Whitehall departments will publish their cuts – sorry, business – plans following last month's spending review. The emphasis will be on reforms rather than cuts. The Coalition's plans on welfare, education and health are more radical than they are given credit for. But its skills will be tested to the full as it tries to sell them.
The Government has won the voters' permission to tackle the deficit. But it is very hard to introduce reforms and cuts at the same time, and there have been mixed messages. Is the Coalition's core purpose to revive the economy, slim down the state or improve social mobility? It will need to offer more than deficit-reduction to avoid the mid-term blues afflicting President Obama.
Despite that, this Coalition is built to last. I would be surprised now if it doesn't survive until the advertised election date in May 2015. I doubt Mr Miliband will get the phone call he wants from Mr Clegg before then.