A former aide to ex-Conservative leader William Hague has defected to the UK Independence Party (UKIP).
Mr Cameron says he does not 'dream of Polly Toynbee'
Toby Horton said his "tipping point" had been a recent suggestion that the modernised party should look to Guardian columnist Polly Toynbee.
Mr Hague's former constituency chairman said: "I didn't leave the Conservative party; the Conservative Party left me."
UKIP has emailed 17,000 UK councillors, urging them to join and is encouraging MPs from other parties to defect.
A spokesman for the Conservatives inisisted the party was not "giving up" its core values, following the Toynbee comments.
Last month, Conservative MP Greg Clark - a key policy adviser to party leader David Cameron - said it was time to focus on ending "relative", not just "absolute" poverty.
It was important to take into account Ms Toynbee's ideas, instead of former Tory Prime Minister Sir Winston Churchill's concept of a "safety net... at the bottom, holding people at subsistence level, just above the abyss of hunger and homelessness".
Mr Horton said: "People are bound to ask me why I have chosen to leave now. The answer, in a nutshell, was Polly Toynbee. That was my tipping point."
Mr Horton, a party member for 40 years, ran Mr Hague's constituency in Richmond between 1996 and 1999.
He stood as a candidate in several elections, including against Tony Blair in his Sedgefield seat.
Tim Mongtgomerie, editor of influential website Conservativehome, recently said that - whatever the merits of her argument - Tories should not be giving Ms Toynbee the "oxygen of publicity".
But in his online video diary this week, Mr Cameron said: "We are not agreeing with Polly Toynbee's policies - let me be absolutely clear about that.
"I do not go to bed at night dreaming of Polly Toynbee, nor do I wake up thinking about Polly Toynbee."
He added: "The point my colleague Greg Clark was making, which I think was a very good point, was when it comes to tackling poverty the old Churchillian metaphor - of saying there should be a ladder up which all should climb and a net beneath which no one should fall - is not enough."
UKIP leader Nigel Farage, who this week told the BBC News website he had turned down the offer of a "very safe" Tory seat before last year said he was "delighted" at Mr Horton's defection.
"This shows that the UK Independence Party is being seen as the real voice of opposition in British politics," said Mr Farage.
The UKIP leader said he was attempting to persuade Tory and Labour MPs and peers who wanted Britain to quit the EU to join his party.
A Tory spokesman dismissed UKIP as" a party of the few dedicated to a single issue".
He said: "The Conservative Party is the only political party that can replace this failing Labour government.
"We are committed to representing modern Britain and working on all issues of concern to the British people. Those who think this means giving up our traditional values are misguided."