Nick Clegg is to step up his warning against Tory plans for tax breaks for married couples - accusing ministers of seeking a return to the 1950s.
In what will be seen as a bid to reinforce the Liberal Democrats' position within the coalition following splits over the EU, the Deputy Prime Minister will use a speech to attack social conservatism.
Tory backbenchers are pressing for an election pledge to introduce transferable tax allowances worth up to £150 a year to be implemented within this parliament.
The move, which was personally championed by Mr Cameron in the run-up to the election, survived into the coalition deal, though with a clause allowing the junior partner to sit out any Commons vote.
Mr Clegg will use a speech on the "open society" to distance his party from the Tories on a range of social issues and will single out the marriage plans. "We should not take a particular version of the family institution, such as the 1950s model of suit-wearing, bread-winning dad and aproned, home-making mother - and try and preserve it in aspic," he will tell the Demos think-tank.
"That's why open society liberals and big society conservatives will take a different view on a tax break for marriage. We can all agree that strong relationships between parents are important, but not agree that the state should use the tax system to encourage a particular family form.
"Conservatives, by definition, tend to defend the status quo, embracing change reluctantly and often after the event."
Mr Clegg will argue that liberal values are more important than ever as the world faces deep economic uncertainty and risks turning inwards. "The danger in the UK is that the forces of reaction and retreat overwhelm our instinct for openness and optimism. That we succumb to fear - the greatest enemy of openness - in these dark economic times."
The Lib Dem leader will make clear that he considers David Cameron's Big Society agenda to be "broadly compatible with the liberal concept of an open society.
But the Tories do not recognise that societies can be "oppressive" as well as the state and that power is better invested in individuals, he will add.