Conservative infighting over the coalition's gay marriage reforms is deepening, as Tory MPs defying David Cameron are told they risk alienating voters.
Pro-reform Tories have penned an open letter pointing out the party faithful applauded Cameron when he made his support for the move clear in his first conference speech as leader in 2005.
Their joint effort is a riposte to the letter, organised by senior Tory opponents to the coalition's marriage (same sex couples) bill, which expressed dismay at the "discord" being created by the legislation.
"We agree that there are many vital issues for the government to tackle. But marriage and how we treat minorities are important issues, too," the letter, signed by 50 senior Tories, states.
"We appreciate the concerns of some of our supporters. But the freedom of faith groups to disagree is protected in the bill. By opposing gay marriage outright, we risk alienating the voters we will need in 2015."
This evening opponents of the bill including Tory MP David Burrowes will meet to outline the basis of their hostility.
"I don't think it makes good politics. Whichever way one looks at it, it's vote defining," Burrowes told politics.co.uk.
"As much as they are bemused by the government prioritising this in legislation and don't see a mandate, they don't see how this will dissuade many supporters.
"The one example it can't be characterised as traditional Tories and the older generation alone who are against. You've only got to look at to where Conservatives want to reach out, to the BME community. Polling indicates this group will mark the Conservatives' card on moral issues."
Only around 120 Tory MPs are expected to vote with coalition ministers in tomorrow's second reading bill, with around 60 set to abstain. The number of Conservative backbenchers supporting the legislation could be even lower than expected, Westminster insiders are suggesting.
Some may be discouraged by informal pressure from the whips, despite Downing Street having earlier conceded a free vote. Backbenchers are reportedly being told their career prospects are under threat ahead of tomorrow's vote.
Some Cabinet ministers may vote against the bill, including defence secretary Philip Hammond, environment secretary Owen Paterson and Welsh secretary David Jones.
Despite the bill containing a 'quadruple lock' which the government claims will protect organisations like the Church of England from legal challenge, incoming Archbishop of Canterbury Justin Welby spoke out against the legislation earlier.
The ResPublica report argues marriage is "inescapably heterosexual" as its primary purpose is to provide a form of social insurance for children and their mothers.
"It cannot be reduced to a contract between the partners, since it is the way in which one generation makes way for and cares for its successor," co-author Professor Roger Scruton argued.
"More than any other institution, society depends on marriage for its future, and no government should meddle with marriage without having the most serious of reasons - reasons far more serious than those so far given."
Writing in the Times newspaper, however, culture secretary Maria Miller acknowledged her party found "easy accord" hard to achieve on the issue.
But she insisted the Conservatives would make "challenging decisions" and "govern in that vein".
"If a couple love each other, then the state should not stop them getting married unless there is good reason — and being gay is not reason enough," she wrote.
"For some individuals and faiths, that statement goes beyond their beliefs and that has to be respected. It is not the role of government to tell people what to believe. However, the state does have a responsibility to treat people fairly."
The controversy may not prove quite the vote-loser for the Tories some in the party are warning. Only seven per cent of voters believe the issue will be decisive in affecting how they vote at the next general election, a poll by YouGov found.