Chancellor George Osborne today gave his strong personal endorsement to plans to legalise gay marriage, and warned Conservative opponents that ditching the policy would be toxic electorally.
The proposals have sparked determined resistance from campaigners for traditional marriage, and many Tory backbenchers are expected to oppose them when the legislation comes before Parliament, where they have been promised a free vote.
But Mr Osborne, who is in charge of Conservative preparations for the 2015 general election, today warned that polls indicate a "clear majority" in favour of the change, particularly among the young and women.
In an analysis of Barack Obama's victory in last week's US presidential election, Mr Osborne said that Mitt Romney's prospects were undermined by the sense that the Republicans were out of step with modern America on social issues such as abortion and gay marriage.
And he invoked Margaret Thatcher in saying that successful political parties must reflect modern life and the way that people want to live their lives.
After a series of senior Tories, including Health Secretary Jeremy Hunt, indicated their personal support for a reduction in the time limit for abortion, Mr Osborne also made clear that he opposes any cut from the current 24 weeks.
Writing in The Times, the Chancellor said: "I should declare my personal position on these social issues: I wouldn't change the current abortion laws and I strongly support gay marriage on principle.
"Of course in Britain these issues are ones of individual conscience and free votes, but I am proud to be part of a Government that will introduce a Bill to allow gay marriage.
"It is worth reflecting that in Britain, as in America, a clear majority of the public support gay marriage, and an even bigger majority of women support it. That majority support is just as high in the North as it is the South, and it is equally high among all socio-economic groups.
"Successful political parties reflect the modern societies they aspire to lead. As Margaret Thatcher said in the first sentence of her introduction to the 1979 Conservative Election manifesto: 'The heart of politics is not political theory, it is people and how they want to live their lives'."
In a challenge to the traditional consensus that economic issues determine election results, Mr Osborne pointed out that the Republicans lost "swathes of voters who were on their side of the economic argument" because of their positions on social issues.
"It is astonishing that Mr Romney won the election among men by a clear-cut margin of 7%; but it was Mr Obama's 11% lead among women that won it for the President, even though many of those that voted Democrat thought Mr Romney would manage the economy better," said the Chancellor.
"President Obama's high-profile endorsement of equal marriage for gay couples also enthused younger voters. But polls found that a majority of all Americans supported him on the issue and voted for it in all four states that held ballots."