Nick Clegg will today face the biggest policy test of his leadership when he asks the party to back radical plan for a £20 billion cut in public spending.

The Liberal Democrat leader launched the policy with an attack on Mr Cameron’s “ill thought-out, juvenile approach” to tax cuts that benefited the wealthy and only trickled down to those less well-off.

“I don't think ordinary people deserve to be trickled on,” Mr Clegg told an audience in Bournemouth.

The plans to reduce tax on middle and lower incomes are central to Mr Clegg’s attempts to move the party to the centre and the vote by members this afternoon is expected to endorse the plans.

Mr Clegg denies this it is an explicit attempt to target Tory seats saying tax cuts appeal to voters across the political spectrum.

A team led by Jeremy Brown, a Treasury spokesman, is currently investigating where in Whitehall spending cuts should fall and has already identified the identity card scheme and the abolition of the Department for Business and Regulatory Reform as areas savings could be made.

The money would be allocated to the Liberal Democrats' own spending priorities and any leftover cash would be used to fund additional tax cuts, on top of a 4p reduction already promised in the basic tax rate.

Opposition is being led by the maverick Evan Harris and Paul Holmes, who are on the left of the party, who want the money to go into public services.

Mr Harris, the party's science spokesman, said that the public had been given mixed signals about the tax cuts, with some people believing all £20 billion would be spent on reducing the tax burden for low and middle-income voters. Dr Harris has tabled an amendment insisting that further cuts should be considered only as a secondary policy.

Mr Harris told the BBC this morning: “We are already calling for huge tax cut on the poorest and close the loopholes - we are debating today on the overall tax burden - we are not calling for more spending , but wiser spending.

“Our party says spend wisely and not tax more.”

The party are lining up a heavyweight trio of Mr Clegg, Treasury spokesman Vince Cable and defeated leadership challenger Chris Huhne to help push through the plans.

Mr Clegg yesterday gave warning that the British public would soon start asking Mr Cameron some difficult questions. “I think they are doing the easy bit now, which is muddying the water, saying they are everything to everyone.

The hard bit is when people will ask him in the next few months: what will you actually do? Are you prepared to make those tough choices?”

Mr Clegg urged delegates to back his plans and “go one step farther” on tax. He said that he wanted to help those who had been really struggling and paying out more of their income in tax than the wealthy.

Mr Cable, the party's economic spokesman, will continue to make a case for tax cuts in a speech today, when he will also outline a six-point plan to generate £5 billion from closing tax loopholes on business and the rich. Mr Cable will say: “The Liberal Democrats are committed to ensuring that families on low and middle incomes pay less tax. However, we must ensure that those at the top are paying their fair share.”

Mr Cable will say that between £10 billion and £40 billion a year in tax is being avoided. Among the proposals are plans to prevent sacked businessmen - such as Adam Applegarth, the former chief executive of Northern Rock - from walking away with tax-free windfalls. Businesses will also be made to pay their fair share of stamp duty when buying property and so called “non-dom” foreign workers will also be made to pay more taxes.

Rather than use the platform yesterday to attack the embattled Gordon Brown, Mr Clegg seemed more intent on doing damage to his Tory counterpart. Defending accusations that he was “Cameron Lite”, Mr Clegg criticised what he called Mr Cameron's “infantile reaction” to last month's conflict between Russia and Georgia.

The Tory leader, he said, “flew off to Tbilisi for a photo-shoot”, adding that it was classic Cameron hypocrisy to demand that Europe present a united front against Russia while at the same time proposing the break-up of the EU.

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