Liberal Democrat leader Tim Farron is pledging to keep up his party's fight against the Government's plans to cut tax credits for millions of low earners.
Following the Government's bruising defeat last month in the House of Lords, Chancellor George Osborne said he would bring forward measures to mitigate the impact of the cuts when he unveils his spending review next week.
But in a speech to the Institute for Public Policy Research think-tank, Mr Farron will say that simply introducing transitional protection will not be enough and he will urge Labour to join with the Lib Dems in opposing the measure.
"The phrase 'hard-working families' has become a terrible political cliche. But in the case of those that will be most hit by the removal of tax credits, it turns out to be spot on," he will say.
"So the Chancellor's plans aren't just unfair, they are also self-defeating and that's why the Liberal Democrats will oppose them every step of the way."
The Lib Dem leader will also urge Mr Osborne to use the spending review to take advantage of the continuing low interest rates to invest in Britain's infrastructure and new house-building.
"If you've got 0% interest rates then the sun is shining. Why don't we fix the roof?" he will say.
Mr Farron, who is seeking to rebuild the Lib Dems after their general election drubbing, will seek to position them as the "pro-enterprise party", with a series of measures to encourage investment and small businesses, including a review of business rates.
"The liberal spirit is the entrepreneurial spirit and entrepreneurs are natural liberals," he will say.
"What does it mean to be an entrepreneur? It means believing that no-one owes us a living, but nor should government get in the way of us making a living for ourselves - like liberals."