Nick Clegg will attempt to restore the morale of shell-shocked Liberal Democrats with a rallying call to remember their "unique mission" to advance the cause of liberal values in British politics.
After his party's hammering in last month's European and local elections, the Deputy Prime Minister will use a keynote speech on Monday to deny they had "lost their soul" in coalition with the Conservatives and to trumpet their role in securing the economic recovery.
Aides said that his address would effectively "fire the starting gun" on an extended 12 month campaign for next year's general election, beginning with a series of key policy announcements over the course of the summer.
A source close to the Lib Dem leader said he accepted the party had been sent a "message" by voters in last month's elections - suffering heavy losses in both the English local councils and the European Parliament, where it slumped to a humiliating fifth place behind the Greens.
"We always knew being in coalition with the Tories would cost us support but we must not let our opponents caricature us - the only reason we are in government is to make Britain a more liberal place and we must show people that," the source said.
"As a result, Nick has decided that now is the time to start setting out the party's election stall in more detail and in many ways this speech fires the starting gun on a 12 month general election campaign."
In his speech, Mr Clegg will defend his decision to enter into coalition with the Tories after the 2010 general election and warn that the Lib Dems will not allow their critics to "airbrush out" their role in delivering recovery.
He will acknowledge, however, that they need to be "much clearer and louder" about their motives and their reasons for wanting to govern.
"I lead the bravest and toughest party in British politics - a party that, back in 2010, despite having never been in power in Westminster before, put the country's interests before our own interests to provide Britain with a stable government in extraordinarily insecure times," he will say, according to an advance extract of his speech.
"There are people out there who believe that the Liberal Democrats are only interested in power for power's sake. Our critics have spent four years claiming we've lost our soul - but it's simply not true.
"There is one reason and one reason only that the Liberal Democrats want to govern. It is because we have a unique mission in British politics - to advance our liberal values.
"The Liberal Democrats are not and will never be a split-the-difference party. I am not interested in coalition at any cost - I am interested in Liberal Democrats in government to deliver a more liberal Britain."
Mr Clegg will reject the idea of the Lib Dems as "some sort of electoral wedge" standing ready to prop up either Labour or the Conservatives in the event of another hung parliament.
Their manifesto, he will say, would offer a "unique and distinct vision" for the country and an alternative to "the depressing prejudices of left and right".
"The thread that runs through that manifesto will be a liberal optimism - the belief that there is something great in every individual and government's job is to help unlock it," he will say.