LONDON (Reuters) - British Prime Minister David Cameron's ruling Conservative party split in two on Tuesday over his plans to legalise gay marriage, a move that many of his own lawmakers said was wrong, not a priority for the public, and unnecessarily divisive.
Many Conservative MPs stood up in parliament to denounce the legislation ahead of a vote in which up to half of Cameron's 303 lawmakers are expected to reject the measure on moral and religious grounds, threatening a corrosive legacy of bitterness.
It was getting into "Alice in Wonderland territory" for any government to come along and rewrite the rules of marriage, Conservative lawmaker Roger Gale told parliament, echoing the views of many in his own party.
"This is not evolution, it's revolution," added Edward Leigh, another Conservative member of parliament, saying marriage was "by its nature a heterosexual union".
Expected to be carried thanks to support from Cameron's political rivals, the vote will take place soon after 1900 GMT.
In a sign of how divisive the issue has become, the finance minister, the foreign secretary and the interior minister implored fellow Conservatives to back the new law.
Cameron is trying to perform a tricky and perhaps impossible balancing act: to reconcile his desire to show his party is progressive, while many of its members are profoundly uncomfortable with such a reform.
Amid talk of a possible leadership challenge to Cameron, many Conservative lawmakers say the prime minister is sacrificing core party values on the altar of populism.
"He hasn't got a lot of political capital left in the bank," Stewart Jackson, a Conservative MP who opposes the gay marriage bill, told Reuters. "He has to deliver some authentic Conservative policies very soon."
Such talk is rife among some Conservative lawmakers and follows a spate of articles in the British press in which a handful of MPs raised the possibility of ousting Cameron, a prospect most commentators regard as far-fetched before the next election in 2015.
The gay marriage initiative has infuriated rank-and-file party activists and a protest letter signed by 25 past and present chairmen of local Conservative associations warned that members were starting to resign over the issue.
Justin Welby, the newly elected Archbishop of Canterbury and head of the world's 80 million Anglicans, used his first comments after being confirmed on Monday to reiterate his opposition to gay marriage.
GRIEVANCES AGAINST CAMERON
Conservative MPs' grievances are many: that Cameron is "arrogant", that he is too fond of the European Union, that the party's policies have been diluted by its coalition partner because Cameron failed to win the last election outright, and a nagging fear that he will not win the next one.
A YouGov poll for the Sunday Times on Sunday showed 55 percent favoured legalising gay marriage, while 36 percent opposed it. However, the same poll showed the issue was not one that concerned most voters.
The new law proposes legalising same-sex marriage in 2014. It would also allow civil partners to convert their partnerships into marriages.
Gay marriage supporters say while existing civil partnerships for same-sex couples afford the same legal rights as marriage, the distinction implies they are inferior.
In a sometimes emotional debate on Tuesday, several gay MPs from different parties took to their feet to commend the bill, describing the prejudice they had suffered growing up.
"Millions will be watching us today," said Nick Herbert, a gay Conservative MP. "Not just gay people but people who want to live in an equal society."
Faced with strong opposition from the Anglican and Catholic churches, the law would not force them to conduct gay marriages, but critics say gay people may launch legal challenges.
Tuesday's vote in the lower house of parliament, the House of Commons, will be "free", meaning MPs will be able to vote according to their conscience rather than under party orders.
WARNINGS OVER DIVISIONS
Supported by Cameron's junior coalition partner, the Liberal Democrats, and by the opposition Labour party, the legislation is months and several stages away from becoming law even, if as expected, it is approved on Tuesday.
Peter Kellner, president of pollster YouGov, said he felt the rebellion would hurt the Conservative party.
"For Cameron, gay marriage is part of his attempt to persuade the voters that his party belongs to modern, 21st century Britain," he wrote on the pollster's site.
"But the divisions that the gay marriages bill has unleashed ... threaten to send an altogether different message: that the Tories are divided, out of touch and prone to quarrel over issues of little concern to most voters."
With the next election still two-and-a-half years distant, there is also a risk that internal party splits over issues like gay marriage could turn what for now is only talk of a possible leadership challenge into the real thing.
"This comes at a difficult time for the prime minister," said Jackson, the Conservative MP. "People in the parliamentary party are wondering why he has got us into this position." (Editing by Giles Elgood)
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