Lib Dem leader Nick Clegg is expected to describe the government as "gutless, heartless and incompetent" in his first UK party conference speech as leader.
Nick Clegg says he will never join a Labour or Tory Cabinet - but will push for a "new type of government" instead.
He will claim Labour's decision to adopt a Tory policy to cut inheritance tax and benefit the rich came at the cost of plans to cut child poverty.
Lib Dem leaders have traditionally made proportional representation the price of a deal with other parties in the event of a hung parliament.
Mr Clegg, party leader for three months, will also turn on the Tories, accusing the party of "sham politics".
But in his first UK conference speech as leader Nick Clegg will demand a shake-up of the whole political system.
He is expected to call for a £25,000 cap on individual party donations.
He will call for a Constitutional Convention to redraw the rules by which Britain is governed.
'Big test'
In his speech to the party's spring conference in Liverpool, Mr Clegg will also attempt to move on from a damaging row over the EU treaty.
Mr Clegg's speech in Liverpool is seen as a chance to reassert his authority after a bruising week in which he lost three of his frontbench team in a row over European policy.
He does not mention the treaty once in his speech, focusing instead on portraying the party as an "anti-establishment" force that can smash the two party system.
If it means walking out of Parliament when the big parties collude against us, I say: fine Nick Clegg
He will also call for a new crackdown on sleaze - with automatic by-elections for MPs who break the rules.
The Conservatives accused Mr Clegg of "flunking his first big test as leader" after 15 of his MPs defied his call for them to abstain in a vote on whether a referendum should be held on the new EU treaty.
And he will propose a £25,000 limit on political donations.
But the Lib Dem leader has insisted he had no option and that the row will not damage the party in the long term.
He will also address the question of alliances with other parties head-on, he will say: "Will I ever join a Conservative government? No.
He has tried to switch focus this weekend to the issues he believes matter more to voters, such as education and health.
"Will I ever join a Labour government? No.
'Anti-establishment'
"I will never allow the Liberal Democrats to be a mere annexe to another party's agenda.
On Saturday he received a boost when the conference voted to back his plans for a more personalised and decentralised NHS - although he was forced to make some concessions.
"But am I interested in building a new type of government. Yes
He has also vowed to wage war on Britain's "sclerotic" political system.
"Based on pluralism instead of one party rules? Yes
He is expected to use his speech later to portray the Lib Dems as an "anti-establishment" party - and pledge to "do whatever it takes" to break the power of the two main parties.
"A new system, that empowers people not parties? Yes. "We want a new, more liberal Britain."
"If it means walking out of Parliament when the big parties collude against us, I say: fine," Mr Clegg will say.
"If it means boycotting banquets that celebrate our relationship with dodgy regimes, like Vince Cable did, or speaking up to expose corruption like Chris Davies did, I say so be it.
"If it means risking court, and refusing to sign up for an identity card, I say bring it on."
Attack
He will call for a cap on individual donations to political parties at 25,000 - half of what is recommended by an independent review, headed by Sir Hayden Philips.
And he will say he wants an end to big money politics, big union donations and offshore finance from Belize, a reference to the Tory donor Lord Ashcroft.
Without such changes, he will warn Britain could end up like America where political influence is all about cash.
He will also stress the party's independence - mounting a scathing attack on his political opponents.
"Remember last autumn, after the election-that-never-was?," he will tell delegates.
"Alistair Darling nicked a policy from the Tories and announced an inheritance tax cut that will help the richest 6% of people. And do you know where they found the money?
"If the rumours are true, they scrapped a plan they'd been secretly developing all summer.
"A plan to cut child poverty. The future of hundreds of thousands of children sold down the river for a cheap political stunt.
"And the Labour backbenchers cheered and whooped.
"Gutless. Heartless. Incompetent."
The Conservatives, he will say, are "a party bereft of belief that will say anything to get elected".
And he will accuse Tory leader David Cameron of taking "a conscious, strategic decision not to have any policies".
