As coalition relations deteriorate as next year's general election nears, Nick Clegg painted a nightmare vision of Britain under majority rule by Conservatives or Labour and urged his "underdog" party to fight "tooth and nail" to prevent it.
Mr Clegg said Britain would be "diminished and divided" after five years of outright Tory government, with a "botched" EU renegotiation and referendum leading to a collapse in market confidence, rising interest rates and a stalled economy and a Conservative leadership "in hock to their right wing" with David Cameron "trapped between a poor man's Margaret Thatcher and a rich man's Nigel Farage".
And he warned that Labour victory would mean more borrowing and debt, a return to recession, soaring unemployment and a generation of young people "thrown back on the scrap heap".
In a plea to be able to act as kingmaker in a second coalition, Mr Clegg said: "We can't allow Labour to throw away our recovery. And we can't allow the Conservatives to put the interests of the few above the needs of the many.
"The only party that can build a stronger economy and a fairer society, so that everyone can have the opportunity to get on in life, is the Liberal Democrats."
As the Lib Dems' last autumn conference before the general election began in Glasgow against a backdrop of dismal polls putting the party on as little as 6%, Mr Clegg admitted they face "the fight of our lives" in next May's poll.
"Once again, we are the underdogs," he told a rally of activists. "Written off. Dismissed. Told that we can't make a difference.
"I don't know about you but I've heard this all before. We have upset the odds before and we will upset the odds again ...
"If we want to stop Labour and the Conservatives from taking Britain backwards, we can. But we have to fight them tooth and nail to do it."
Mr Clegg asked delegates to imagine what Britain would be like in 2020 after five years of a Tory-only government: "Britain, diminished and divided after a botched attempt to renegotiate our relationship with Europe and a vote to withdraw from Europe; companies pulling out of the UK left right and centre; the markets losing confidence, hiking up our borrowing costs and halting the recovery in its tracks.
"Workers fearing for their jobs, not just because the companies they work for are plunged into uncertainty but because their bosses can fire them at will, no questions asked. The young and the working poor hit time and time again as George Osborne takes his axe to the welfare budget with no regard for the impact on people's lives."
Schools would be run in the interests of shareholders' profits, the Home Office would snoop on emails and social media, and opportunity "reserved for the few at the top", he said.
And he said the Tory leadership would be "in hock to their right wing, desperately running after and pandering to Ukip's ugly nationalism", with "'compassionate conservatism' just a soundbite from a bygone age".
Meanwhile, Mr Clegg said a Labour government led by Ed Miliband would mean "a Labour chancellor with his hands on the nation's credit card, borrowing a few million here and a few billion there, throwing good money after bad at every problem that comes along."
Labour governing alone would mean "debt piling up and up until the day of reckoning comes, a recovery squandered, recession. Unemployment soars again, especially for the young. Fear, insecurity, opportunity stifled, hopes and aspirations killed stone dead.
"A generation thrown back on the scrap heap just when it looked like they had reason to be optimistic again, our children forced to pay for the mistakes of the generations before them".
Mr Clegg did not indicate which party he would prefer to go into coalition with in the case of a hung parliament, but said he was "proud" of the Lib Dems' record of action in coalition with Tories over the past five years, highlighting their part in introducing free infant-age school meals, the pupil premium for disadvantaged children, an expansion of apprenticeships, gay marriage, shared parental leave, the triple-lock on pensions and the rise in the lower threshold for income tax.
If they stay in government after next May's poll, the party would introduce free meals throughout primary schools, offer free bus travel to young workers, extend paternity leave and free childcare, eliminate the deficit and guarantee "proper" funding for the NHS, he said.
"More opportunity, more freedom, fewer barriers," said Mr Clegg.
"I am proud of our record of action and I want us to be in government again so that we can do more, so that we can continue to build a stronger economy and a fairer society, where everyone has the opportunity to get on in life."