Britain's Prime Minister David Cameron addresses a news conference in Brussels on May 22, 2013 After building a coalition by embracing liberal social issues, Cameron is facing conservative backlash. (FRANCOIS LENOIR/REUTERS)

Republicans weighing a shift to the center ahead of the next presidential race will find a cautionary tale on this side of the Atlantic, where British Prime Minister David Cameron is paying the price for his foray outside of strictly conservative territory.

Cameron strode into No. 10 Downing Street three years ago with a platform that included die-hard conservative values such as slashing the deficit and shrinking big government. But casting himself as a new breed of conservative that could appeal to Britons of every stripe, he also went out on a political limb — embracing social causes such as same-sex marriage and the fight against climate change.

That limb is dangerously cracking. Cameron is coping with the fallout of a cleaver attack by Islamist extremists on Wednesday that left a British soldier dead and put the nation on edge. But far more important to his political future, a long-simmering rebellion from the right flank of his Conservative Party has boiled over in recent weeks, sparking what some grass-roots campaigners here are openly calling a conservative “civil war” that underscores the risk for any politician who challenges his party’s base.

In the latest evidence of the kind of internal revolt that ultimately forced Margaret Thatcher from office in 1989, a powerful group of conservative legislators this past week sought to sabotage one of the pillars of Cameron’s plan to broaden his party’s appeal — same-sex marriage. After a heated debate on the floor of the House of Commons, more conservatives voted against the bill than for it, forcing Cameron into the humiliating position of having to depend on the opposition Labor Party to push the legislation through the lower chamber.

The discourse over same-sex marriage included such right-of-center voices as Lord Norman Tebbit, who warned of the risk of a “lesbian queen” in Buckingham Palace who could bear heirs via artificial insemination. The right-wing mutiny is also unfolding on multiple other fronts, from Britain’s role in European affairs to government support for alternative sources of energy.

Conservatives defecting

A gifted verbal jouster and Oxford man, Cameron, 46, led the Conservatives in 2010 to their best electoral performance since the early 1990s — a feat his supporters say could never have happened had he not embraced issues such as gay marriage that now enjoy overwhelming support in Britain. But a growing number of Conservatives are questioning whether Cameron’s shift to the center has truly worked in the party’s favor.

Recent polls, for instance, suggest the prime minister has had very limited success at winning over non-Conservative voters despite his centrist approach. Once-loyal Conservatives, meanwhile, are defecting en masse. Many are heading into the open arms of the newly strengthened United Kingdom Independence Party, a nationalist political force with a platform that stands significantly to the right of the Conservative Party.

“Old fossils like me always wondered about this great shift to the promised land of the center,” said Lord David Howell, who served in Thatcher’s conservative cabinet in the 1980s. “Is it really just a journey to a place where no one really likes you anymore?”

The inner party struggle is weakening Cameron’s government, distracting Washington’s staunchest ally from a host of international issues, including the crisis in Syria.

To quell the chorus, Cameron has given in to at least one major demand by his right wing. In January, he pledged to hold a historic referendum by 2017 on whether Britain should break away from the European Union, an institution reviled by many on the right. He is under pressure by Conservative rebels to go even further, either moving up the timing of the vote or passing legislation that would set such a referendum in stone.

The referendum — opposed by the Obama administration, which fears that a British exit from the E.U. could diminish the voice of its strongest ally in Europe — is only part of a bigger Conservative uprising. In the House of Lords, Britain’s upper chamber, more conservatives are vowing to break with Cameron and oppose the gay marriage bill when it lands there next month. Cameron is also weathering scathing criticism from Conservatives deeply opposed to government support for onshore wind farms.

Stoking the fires further were allegations this past week that a close aide to the prime minister had described angry grass-roots Conservatives as “swivel-eyed loons.” Although the comment was denied by Cameron’s camp, it nevertheless highlighted the very real sense among rank-and-file Conservatives that their leader is an elitist, disconnected from the party’s base.

“David Cameron thought he could move the party in this direction, and if he lost a few people along the way, too bad,” said Geoffrey Vero, a Conservative campaigner in Surrey, about 30 miles south of London. “But he is losing more than he bargained for on the ground, and he isn’t picking up any new friends elsewhere. A lot of conservatives are looking around right now and saying, ‘I’ve had enough of David.’ ”

Record low approval ratings

Analysts say Cameron is not in immediate jeopardy of being ousted by his own party a la Thatcher. His potential rivals are facing almost as many problems as Cameron himself. But there are dangerous months ahead.

If UKIP tops the Conservatives in elections for the European Parliament next year — as some now predict — the chorus of dissent that Cameron is facing may grow to the point that his position at the head of the party is undermined.

Analysts also say it is unfair, when assessing Cameron’s hold on power, to discount the hard-to-measure success he may have had with centrist voters. They note that Conservatives only managed to break a 13-year exile from No. 10 Downing Street in 2010 by forging a coalition deal to rule alongside the more progressive Liberal Democrats.

And many argue that Britain’s troubled economy probably has more to do with Cameron’s steady loss of popularity with the public — some polls show him with record low approval ratings around 26 percent — than any of his social polices.

“The fact is, previous Conservative candidates who all ended up playing the right-wing card have ended up losing elections or the leadership of the party,” said James Hanning, co-author of the biography “Cameron: Practically a Conservative.”

Those Conservative leaders “kept the voter base happy, but that’s not what the rest of the country wants,” Hanning said.

And yet, there are signs that Cameron, in a quest for survival within his own party, may be ejecting some of his centrist ideals. “When is the last time you heard him give a speech on green energy?” Hanning said.

After the bruising fight over same-sex marriage this past week, Cameron sought to reassure his base that he would refocus his attention on Britain’s core economic issues and budgetary reforms.

“To anyone who doubts the life there is left in the coalition, I would argue there is more to come,” he told the BBC. “Very bold reforming and strong government, and that is what we’ll be right up until polling day."