Choosers needn't be beggars. On that corollary of the well-known maxim rests the Liberal Democrats' strategy for political survival.

The party is strapped for cash and starved of popularity. Its only hope of escaping political penury lies in parliament staying hung after the next election. The notional power to choose between Labour and the Tories, in effect nominating the prime minister, is all the riches Nick Clegg needs in an age of coalition.

In that respect, the outlook for the Lib Dems isn't as bleak as their poll ratings suggest. Westminster is deadlocked. David Cameron and Ed Miliband both struggle to describe a plausibly optimistic route out of austerity. The Tory leader is pinned back by voters' visceral mistrust of his party; Labour's advance is blocked by instinctive rejection of its leader.

In theory, that opens a niche for Clegg to combine the authority of an incumbent, which Miliband lacks, with a claim to have diluted the more pungent strains of Tory ideology. In practice, he is miles away from being that candidate. The glimmer of hope in grim opinion polls is evidence that more people are glad the Tories hooked up with the Lib Dems in 2010 than wish Cameron could govern unbound.

That, say aides, proves there is mileage in the plan to carve out an identity for the party as the kindly face of the coalition. Much fuss was made in Lib Dem HQ of a choreographed "trio of speeches" that Clegg made in December and January - on social mobility, employee ownership ofcompanies and tax breaks for low earners - each of which generated benign headlines for the Deputy Prime Minister, coinciding with a modest revival in his personal poll scores.

This accelerated campaign of Lib Dem "differentiation" has not passed unremarked by the Tories. Downing Street's default response when Lib Dems are briefing their policy triumphs to the media is icy condescension. "I don't think it helps them. It just makes them look weak," says a No 10 source. But patience is wearing thin. "They need to be careful," warns one Cameron aide.

Battle of the brands

This, say Lib Dems, shows that the Tories are rattled, knowing their brand is vulnerable. Increased tension is held up as proof of an effective transition from one style of partnership - the lockstep intimacy of the first years - to something looser. "It isn't the cordial coalition any more," says a party strategist. That much is confirmed by civil servants who witness the daily management of coalition relations. "It has become more zero-sum, more spiky," observes one senior mandarin.

The Lib Dems, meanwhile, claim to be flattered by recent shows of strength by the Conservative back benches. On 5 and 6 February, letters signed by more than 100 Tory MPs were published urging the government to change policy, first by opposing windfarm subsidies, then by rejecting European integration in criminal justice policy. The message was that there are many more Tory backbenchers than there are Lib Dems in parliament and that the balance of influence over the coalition's programme should be calibrated accordingly.

This burst of blue-handled sabre-rattling came at a conspicuous moment of Lib Dem weakness: days after the departure from cabinet of Chris Huhne, who faces criminal charges relating to a driving offence. The former energy secretary will not be much missed by Clegg - he was a rival for the leadership in 2007 and had been a periodic irritant ever since. Still, the prospect of a court case and the attendant soap opera (Huhne's broken marriage being central to the plot) is grim. It threatens to generate a lot more publicity for the Lib Dems than Clegg's Whiggish disquisitions on fairer capitalism. Even if Huhne is acquitted - and he must be presumed innocent - "differentiation" risks being trampled into the formless mulch of public scorn for all politicians.

That won't stop Tory MPs angling for differentiation of their own. Backbenchers have been made nervous by rumours that Cameron is entertaining the idea of prolonging coalition beyond the next election. The whisper around Downing Street is that the Prime Minister can envisage recruiting the Lib Dems as a semi-permanent brake on the Conservative Party's habitually disobedient right-wing fringe.

Partly that chatter reflects sincere recognition of the electoral arithmetic that makes it hard for the Tories to win a majority. Yet senior Lib Dems suspect a more sinister motive, which is to undermine Clegg's independence by portraying the choice he made in 2010 as irreversible. Seeding the idea of the current coalition continuing after the next election reinforces the impression that the junior partner has been annexed, which helps smother in advance any talk of collaboration with Labour.

Beggar's opera

Not that any such partnership is in imminent prospect. Relations between the two leaders are civil but perfunctory. Strategic calculations drive them apart. The Lib Dems need to trash Miliband's credibility on the economy in order to justify their alliance with the Tories. Labour needs to belittle Clegg's influence in government to monopolise the politics of fairness and compassion.

The Labour front bench seems to operate a policy of wishful decapitation towards the Lib Dems. Shadow ministers declare themselves able to imagine one day working in coalition with the third party, before adding the mandatory caveat - "but not with Clegg". The assumption is that the only credible way the Lib Dems can distance themselves from the Conservatives is to ditch the leader who negotiated alliance with them.

But for the Lib Dems to punish Clegg for cosying up to Cameron would repudiate the concept of coalition as a way of doing politics that rises above tribal vendetta. Without that ethic, the party has nothing.

Parliament is on course to remain hung. There lies hope for the Lib Dems as prospective powerbrokers. The problem is that the Tories and Labour are conspiring - the former by design, the latter more by accident - to close down Clegg's future negotiating options; and if the Lib Dem leader is robbed of the prospect of one day campaigning as a coalition chooser, he is politically beggared.