David Cameron will not win the general election merely by dismissing Ed Miliband as a "1970s socialist", Margaret Thatcher's advertising guru has warned.
Lord Saatchi, a former Conservative Party chairman, cautioned against underestimating the potency of the Opposition leader's argument on wealth inequality.
The Prime Minister has repeatedly deployed a "same old Labour" attack line against Mr Miliband, and in his conference speech earlier this month vowed to crush his "1970s-style socialism".
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But writing in the Mail on Sunday, Lord Saatchi said Tories should not be so quick to welcome Mr Miliband's apparent shift to the Left.
"Many Conservatives rejoice at this news, that Labour is again about to sign 'the longest suicide note in history'," the peer wrote.
"Is that true? It's been 21 years since the Conservative Party won an Election.
"You hear it said that the party was unlucky to have a succession of five leaders with insufficient appeal to voters. That seems statistically unlikely.
"A more plausible explanation is that the party has lacked a marching tune people can respond to. This might be because it has underestimated the power of socialism."
Lord Saatchi said people had "gone off" socialism in the 1980s because it did not create wealth, but now capitalism was also seen as flawed.
"Now we don't know what we like," he added. "That is probably why, on so many key political issues, when the public is asked which party has the best policies, the answer is often 'neither'."
Lord Saatchi said the last few decades had seen the creation of "global cartels" in areas such as banking and energy, where there was a "huge imbalance of power between the individual customer and the giant corporation".
"The overwhelming power of money in such a climate is a dangerous moment for Conservatism," the peer wrote. "What scares people most is soon money will talk in health as well as everything else...
"People may conclude they need someone to protect them from that kind of 'free market', such as, perhaps, the state. This is why Labour thinks they have struck gold with a state price freeze on energy.
"Conservatives like to say state socialism is a return to 1970s failure.
"But will that do? Perhaps not. Because one thing has changed radically since the 1970s, which is that the new economic superpower of the world is itself a socialist state."
Lord Saatchi said he was sure that Mr Cameron and Chancellor George Osborne "fully appreciate the intellectual challenge of this new world order, and will not stubbornly insist on the existing version of free-market capitalism as the only way forward.
"That would open the door for Labour to access millions of people, especially young people, who do not accept the status quo."