The coalition's long-trailed consultation on gay marriage finally begins today. And the outcome, it appears, has been largely pre-determined. As Lynne Featherstone, the equalities minister, who is leading the consultation, tells today's Independent , "The essential question is not whether we are going to introduce same-sex civil marriage but how." Elsewhere, in an op-ed  for today's Times (£), Theresa May becomes the latest senior Conservative to declare her support for the proposal, making the sound argument that marriage, a social good, should be extended to as many people as possible.
With the support of so many cabinet ministers, it's hard to see gay marriage not becoming law by 2015. For David Cameron, desperate for his government not to be defined by deficit reduction alone, this is a chance to effect lasting social change.
But he will face significant clerical and parliamentary resistance. The government has already agreed to give Conservative ministers, some of whom are prepared to resign over the issue, a free vote in the Commons. Defence minister Gerald Howarth , for instance, has already clumsily declared his opposition to gay marriage: "Some of my best friends are in civil partnerships, which is fine, but I think it would be a step too far to suggest that this is marriage".
Then there's the church. The government has already ruled out making it compulsory for religious organisations to host gay marriages but that hasn't placated the faithful. Cardinal Keith O'Brien, the leader of the Roman Catholic church in Scotland, has shamed himself  by comparing same sex marriage to slavery, while Rowan Williams has argued  that the law cannot be used to impose cultural change, and cannot run ahead of public opinion.
Williams is right: more of the public are opposed to gay marriage than in favour of it. But the gap is not as great as some imagine. As I noted earlier this week, according to a recent YouGov poll , 43 per cent of voters support gay marriage , with 47 per cent opposed [32 per cent of whom support the current alternative of civil partnerships] and 10 per cent undecided. Worryingly for Cameron, however, while 51 per cent of Labour voters and 53 per cent of Lib Dems support same sex marriage, just 30 per cent of Tories do. I know of one pro-gay marriage Conservative MP who missed church on Sunday for fear of being accosted by parishioners. The concern among some Tories is that UKIP, explicitly opposed  to gay marriage, will provide a welcome home for any would-be defectors.
And should Cameron change the law, he may not receive much credit for doing so. The YouGov poll I mentioned earlier revealed that 63 per cent of votes think that the PM supports gay marriage for purely "political reasons". Only 21 per cent think that he "genuinely believes that is the right thing to do". The greatest challenge for Cameron, then, is to convince the public that he is acting out of principle, rather than political expediency.