Some ideas in politics are so bad that even disavowing them is getting too close. The notion of the Tories forming an electoral pact with the UK Independence Party is one. For Downing Street to mention it at all, even in a rebuttal, can serve only to advertise Ukip as the natural home for Conservatives who don’t like David Cameron and thereby accelerate the exodus.

Nigel Farage, Ukip’s leader, is an affable pub demagogue who can snaffle stray votes across the political spectrum. That is not a reason for a governing party to put him on a joint ticket. Yet some Tory MPs seriously entertain the idea. Eight are said to have held talks on defection.

This fringe flirtation springs from ideological affinity. Farage says aloud things about Europe and immigration that many Tories think but feel are taboo in Cameron’s party. When the Tory leader described Ukip as a haven for “loonies, fruitcakes and closet racists” in 2006, he told a number of his MPs, in effect, that their career prospects were over on his watch. That left them with three options: quit the party, agitate for new leadership or use the threat of rebellion as a remote control for steering Tory policy from the back benches. Talking up Ukip is a device for “keeping Cameron honest”, as one Tory MP puts it.

Shire calling

The problem, as Cameron knows, is that the Tories failed to win a majority in 2010 because too many voters imagined them a bit too Faragey already – all claret-faced in disgust at modern Britain. It is hard to see how inserting the real Farage into their 2015 manifesto will change those sceptical minds.

Dissident Conservatives argue that the economic crisis has changed the political landscape. There is, they say, a new appetite for right-wing populism and departure from the EU, as shown by opinion polls and Ukip’s harrying of mainstream parties in by-elections. The flaw in that argument is that it doesn’t distinguish between what voters feel on single issues and how ardently they feel it. Europhobia doesn’t damage the Tories because voters love the EU. They don’t. It hurts them by signalling a relapse into manias from the party’s most unattractive years.

There is a sad irony in Conservatives’ fantasies of congress with a party to make them look embittered and extreme, when they are already in a coalition that might – had relations been better managed – have made them look reasonable and moderate.

There is little public affection for the Liberal Democrats. Nick Clegg’s personal ratings mine depths from which few candidates return. Yet the party’s morale is surprisingly sturdy. That is because the Tories make it so easy for Clegg to present himself as a brake on their intemperate urges. Every tender glance that Cameron’s MPs throw at Ukip is a tactical boost for the Lib Dems. Their survival in parliament depends on holding seats in southern England where the nearest rival is a Tory. That means appealing to liberal-minded voters who shudder at the prospect of unalloyed Conservative rule but have written off Labour, either because it is too far behind locally or because it remains unforgiven for its record in office.

In the Deputy Prime Minister’s office, Cam­eron’s failure to rein in Tory MPs is viewed with pity verging on ridicule. Senior Lib Dems believe that the Prime Minister indulged his party’s fanatical tendency for tactical gain in early coalition power struggles – over electoral reform, for example – without understanding the monster he was creating.

“They really are the British Tea Party,” says one Clegg ally. “They’ve been allowed to grow because they once served a useful purpose but now it’s out of control.”

Lib Dem attitudes towards the Tory leader have hardened in frustration over negotiations ahead of the Chancellor’s autumn statement on the economy on 5 December. The Prime Minister is the obstacle to Clegg’s ambition to introduce wealth taxes – specifically a levy on expensive property. George Osborne, chastened by the fallout from his disastrous cut to the 50p top tax rate, is more alert to the hazards of looking cosy with the rich. Cameron remains adamant. His coalition partners say that this comes down to “shire Tory instincts” and loyalty to mansion-dwelling chums. “They’re the people he meets at dinner parties in Witney [the Prime Minister’s Cotswolds constituency],” says a Lib Dem cabinet minister. “And he wants to protect them.”

With levels of respect so low, it is surprising that the coalition works as well as it does. Each side accuses the other of being leaky, leading to petty complaints about the number of advisers in the room when sensitive matters are discussed. More decisions are passed up for negotiation in the quad – the coalition cockpit where Cameron, Clegg, Osborne and Danny Alexander sit. That allows more junior Tories and Lib Dems to go about their business without conflict or, indeed, without seeing much of each other. “You can almost pretend you’re not in a coalition,” jokes a Tory aide.

True colours

Despite the ebbing away of trust, the coalition has found a certain functional equilibrium. It is an arrangement that flatters neither side but it suits the Lib Dems slightly better. They are generally more relaxed about admitting to concessions and imperfections in their deals because they are in the coalition business for the long term and want to showcase it as a decent way to run a government. They also have an interest in being seen to be thwarting Tory ambitions. Most Conservatives have yet to discover the joy of compromise and still resent sharing power, having forgotten, it seems, the voters who decided that they couldn’t be trusted with power unshared.

The crucial difference is that most Lib Dems are enjoying being in government while a solid rump of Tory MPs are not. Their eyes are drawn to the lush political pasture on the side of the fence where Ukip grazes. There they see ideological purity and the chance to express their true opinions. It looks liberating, empowering. It is the siren lure of opposition.