"We are the One Nation party," Nick Clegg will tell the Liberal Democrats in his speech to their spring conference tomorrow. It is meant primarily as an assertion of the Lib Dems' identity – a party that represents all sections of society and all parts of Britain without being in hock to vested interests, while the Conservatives rely heavily business donors and Labour is dependent on the unions.

But Mr Clegg knows his spot of plagiarism will also get under David Cameron's skin. "One Nation" is a Conservative theme dating back to 1845 and Disraeli. Mr Cameron would doubtless prefer his Coalition partner to stick to Gladstone. He wants to be seen as a "One Nation" leader. Mr Clegg implies that, whatever Mr Cameron's instincts, the Tories have not yet abandoned what Disraeli would have called the "two nation" philosophy of Thatcherism.

Mr Clegg's invasion of Tory turf shows how the Coalition game has changed. Its unity phase, which the Tories hoped would last until 2015, is over. The ugly word on the Lib Dems' lips is "differentiation" and they are enjoying their new freedom to differ from their Coalition partners. The Tories hate it. The Lib Dems are even having a bit of fun. When Nadine Dorries, the outspoken Tory MP, attacked the concessions given to the Lib Dems on the Government's health reforms, Richard Reeves, a senior Clegg strategist, sent her a bunch of flowers as a thank you.

Last year, in the unity phase, the Lib Dem leadership told party rebels: "We cannot be government and opposition at same time." Today they try to have it both ways. The Lib Dems negotiate in public over this month's Budget, to George Osborne's dismay. Yet the Lib Dems cannot let disunity push the Coalition off the rails. So they happily work on a joint policy statement for this summer highlighting the issues where they broadly agree with the Tories.

Last year, Mr Clegg would have shared the Tories' anger at this week's leaking of the letter by Vince Cable, the independent-minded Business Secretary, saying the Coalition lacked a "compelling vision" beyond its spending cuts. The leak was not part of a differentiation master plan; the Lib Dems are not that Machiavellian. But it dovetailed with Mr Clegg's strategy, as it showed that the Lib Dems were fighting their corner, were thinking hard and were not the same as the Tories, so the Deputy Prime Minister was relaxed. "It helps us," one ally said.

Their new buzzword is "JEET", which stands for the issues they want to talk about – jobs, education, environment and tax.

But many Lib Dem activists at this weekend's conference in Gateshead fear the party will be dragged down again unless it kills the health reforms. Grim comparisons are made with the impact of university tuition fees on the Lib Dems, the poll tax on Margaret Thatcher and the Iraq war on Tony Blair. If Labour succeeds in making the NHS a big issue at the next election, the Lib Dems will have nowhere to hide, the party's "kill the Bill" brigade warn. There will be no escaping the massacre implied by the current opinion polls.

Mr Clegg's "One Nation" claim is a declaration that the Lib Dems have pitched their tent in the centre ground and hope to colonise as much of it as possible. They do not dream that millions of voters will suddenly flock to them; Mr Clegg seeks a gradual, grudging respect that his party is trying to do the right thing for the country and admits it sometimes gets it wrong.

For this strategy to work, and to prevent a Lib Dem meltdown in 2015, he may need Labour to veer left under Ed Miliband and for Mr Cameron to appease his MPs and Tory-supporting newspapers by turning right to differentiate himself from those irritating Lib Dems. Mr Miliband and Mr Cameron would both dismiss such a course as fantasy, yet their actions sometimes give the Lib Dems hope that it could become reality. Mr Clegg's party cannot be written off yet; it is still in the game.