Americans watching the latest push for social change in Britain might feel as if they had stepped into an alternate political universe: Here, the Conservatives are leading the charge for same-sex marriage.

Gay couples in Britain won the right to civil partnerships in 2004, which granted them nearly the same legal status as married heterosexual couples while avoiding the controversial use of the word “marriage.” But Prime Minister David Cameron and his Conservative-led coalition have launched a historic drive to grant gay men and lesbians the option of also entering into civil marriages, touching off a surprisingly fierce uproar in largely progressive Britain and fueling a rebellion on the right as the party comes under heavy fire from traditional allies in the British clergy.

Yet challenging tradition appears to be exactly Cameron’s point. The proposal, put forward this month despite the lack of a strong clamor for marriage within Britain’s gay community, is nevertheless emerging as the cornerstone of a bid by the 45-year-old prime minister and other young leaders on the right here to redefine what it means to be a modern Conservative.

“I don’t support gay marriage despite being a Conservative,” Cameron said in a recent landmark speech on the issue. “I support gay marriage because I am a Conservative.”

Spurred to action by a book about a child with two dads, the Conservatives under Margaret Thatcher rushed a ban through Parliament in 1988 forbidding local governments and schools from promoting homosexuality, with same-sex couples then described by law “as a pretended family relationship.” Twenty-four years later, strategists see Cameron’s decision to champion the gay marriage cause as an attempt to seize the mantle of progressive change from the left and broaden the Conservative Party’s appeal among an increasingly key voting group: young urbanites.

To be sure, since returning to power in 2010 after 13 years in the political wilderness, the Conservatives have pursued causes at the core of their founding beliefs: slashing the deficit, cutting public payrolls and moving to lower taxes. Yet the party of Thatcher has also sought to reinvent itself by becoming what one Conservative strategist called “very pro-gay.”

There are at least 12 openly gay members of Parliament from the Conservative Party, more than all other British political parties combined. A majority of those lawmakers were ushered into office with Cameron in 2010. Education Minister Michael Gove, a Conservative, has launched a campaign with the gay rights group Stonewall to combat homophobia in British schools. Cameron has hosted a summit on homophobia in professional soccer and officially apologized for Thatcher-era anti-gay policies, calling the party’s previous stance “a mistake.”

What prompted the shift? “We lost three elections, in 1997, 2001 and 2005,” said Margot James, former vice chairman of the Conservative Party and an openly gay member of Parliament.

“The electorate was not seeing us as a viable alternative in a modern world. But David Cameron came along and changed all that. This is a different Conservative Party now, one that is fully in favor of equal rights. I think the Republicans could learn a lot from us in how to appeal to the center, without whose votes a party cannot hope to win.”

Religious dissent

In Britain, legalizing gay marriage would be partly symbolic. Civil partnerships — passed under Prime Minister Tony Blair’s Labor government — gave same-sex couples equal access to national pensions, inheritances, tax breaks and other rights enjoyed by married heterosexual couples. But Cameron, a Christian and married father of three whose position on same-sex marriage gradually evolved since winning the party’s leadership in 2005, is calling gay marriage a matter of basic human rights.

He is also making a pitch to uneasy religious conservatives, suggesting that the institution of marriage will reinforce traditional values of commitment and monogamy within the gay community. Married same-sex couples, for instance, could file for divorce on the grounds of adultery — a legal option not currently considered in civil partnership laws.

The terms of political debate here remain different than in the United States, where the Republican Party base contains a highly influential religious right whose views on social issues are considered extreme even among many British Conservatives. But even here, the notion of altering the definition of marriage — as opposed to granting civil partnership rights — is hardly a safe political bet, with the push generating far more discord than most had anticipated.

Although a new law would not compel churches to perform religious wedding ceremonies for same-sex couples, Cardinal Keith O’Brien, head of the Catholic Church in Scotland, called the proposal “grotesque,” throwing his weight behind a growing national campaign to defeat the measure. The Church of England has decried the idea as tantamount to legislating cultural change and has mounted a rigorous defense of marriage “between a man and a woman.”

The issue is also setting up the biggest internal party rebellion since the Conservatives returned to power in a coalition with the Liberal Democrats. Strategists say that as many as a third of Conservative lawmakers may vote against the plan, which might nevertheless pass in Parliament with the backing of Cameron’s Liberal Democrat coalition partners and the opposition Labor Party.

Debate in gay community

By offering the proposal, Cameron has put his party out in front of even many gay advocates here who had seen other issues, such as stiffer penalties for hate crimes, as higher priorities. Although gay groups are vigorously campaigning in support of the measure, advocates initially debated whether they should even endorse a proposal seen by some as bringing a patriarchal and archaic institution to same-sex couples.

“This is more of David Cameron trying to drag the Conservatives kicking and screaming into the modern world,” said Ben Bradshaw, a ranking Labor lawmaker who in 1997 became one of Britain’s first openly gay members of Parliament. “Of course, we’ll support it, but this is pure politics on their part. This isn’t a priority for the gay community, which already won equal rights” with civil partnerships.

He added: “We’ve never needed the word ‘marriage,’ and all it’s done now is get a bunch of bishops hot under the collar. We’ve been pragmatic, not making the mistake they have in the U.S., where the gay lobby has banged on about marriage.”

But a growing chorus of voices in the gay community here is also livid that advocates have not pushed marriage to the top of the same-sex agenda sooner.

“Now that we’re having this debate, the gay community is failing to step up to the plate and defend our right to marriage as aggressively as they should in the face of this sudden religious opposition,” said Ewan Watson, 37, a London-based lawyer who is in a same-sex relationship. “For some of us, having marriage is huge. It’s a statement that we really are equal, and we won’t be as long as they segregate us from the word ‘marriage.’ ”