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.”…

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.