By Alex Stevenson
Nick Clegg has wasted no time in picking his next fight with David Cameron, laying into the prime minister over his party's "1950s" view of marriage and family values.
The Liberal Democrat leader is to highlight the difference between his party's views on marriage tax breaks, which the Tories support but Lib Dems oppose, in a speech at the Demos thinktank tomorrow.
Mr Clegg will contrast the attitudes of "open society Liberals" and "big society Conservatives" when it comes to the question of whether the state should use the tax system "to encourage a particular family form".
"The institutions of our society are constantly evolving," Mr Clegg is expected to say.
"Just look at the way the roles of men and women, and attitudes to marriage and divorce, have changed over the last century. We should not take a particular version of the family institution, such as the 1950s model of the suit-wearing, bread-winning dad and aproned, homemaking mother, and try to preserve it in aspic."
The deputy prime minister's inflammatory language opens up another front in the very public spat between the coalition's two parties, which has seen internal tensions rise to new levels over Europe.
David Cameron's use of the veto, leaving Britain outside a group of 26 EU member states who will work together on fiscal integration, has enraged the Lib Dems.
Mr Clegg said the summit's outcome was "bad for Britain" before his parliamentary party abstained from a Commons motion praising Mr Cameron.
Tomorrow's speech will see the deputy PM use marriage to highlight the differences between the two parties once again.
He will add: "Open society liberals are progressive: we believe that the future can and ought to be better than the past. Conservatives, by definition, tend to defend the status quo, embracing change reluctantly and often after the event."
Mr Clegg's speech contrasts with Mr Cameron's address on the King James Bible on Friday, in which the prime minister appeased traditionalists by claiming Britain remained a "Christian country".
"We should not be afraid to say so," he said. "What I am saying is that the Bible has helped to give Britain a set of values and morals which make Britain what it is today."
Much of the prime minister's rhetoric in 2011 has been about Britain's "broken society" after rioting in England's biggest cities in August.
Now the Conservative leader is seeking to highlight the "values and morals" of religion in the run-up to Christmas.
He added: "One of the biggest lessons of the riots last summer is that we've got stand up for our values if we are to confront the slow-motion moral collapse that has taken place in parts of our country these past few generations."