LONDON — US 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 diehard conservative values like slashing the deficit and shrinking big government.
But casting himself as a new breed of conservative who could appeal to Britons of every stripe, he also went out on a political limb — embracing social causes such as gay marriage and the fight against climate change.
That limb is dangerously cracking, with Cameron now mired in one of the deepest crises of his tenure. Cameron is coping with the fallout of the recent cleaver attack by Islamist extremists 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 challenging 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 recently sought to sabotage one of the main pillars of Cameron’s plan to broaden his party’s appeal — gay 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 saw right-of-center voices like Lord Norman Tebbit warn 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 energies.
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 like 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 (UKIP), a nationalist political force with a platform that now 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 ongoing crisis in Syria.
To quell the chorus, Cameron has already 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 now 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 is opposed by the Obama administration, which fears a British exit from the E.U. could diminish the voice of its strongest ally in Europe. It is only part of a bigger Conservative uprising.
In the House of Lords — Britain’s upper chamber, which hears the measure next — more conservatives are vowing to break with Cameron and oppose the gay marriage bill when it lands there. Cameron is also weathering scathing criticism from Conservatives deeply opposed to government support for onshore wind farms.
Stoking the fires further were recent allegations 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.’ ’’
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.