John McCain called for an end to the 27-year ban on offshore oil production as he risked tainting his green credentials with an attempt to tap the fears and resentment of American voters over $4-a-gallon (£2) petrol prices.

The Republican presidential nominee’s speech in Houston triggered a swift reaction from his Democratic opponent, Barack Obama, who accused him of betraying previous promises on the environment.

Both sides recognise that the 74-per cent increase in prices at the pump over the last 18 months has been one of the biggest contributors to a growing sense of insecurity for voters who grew up on cheap fuel.

Although petrol costs might still seem low compared to those in Britain — where prices are at least twice as high — the attachment of Americans to their cars and the distances driven daily by suburban or rural families has sent a shock of pain across the political spectrum.

A Washington Post/ABC poll yesterday showed 80 per cent of voters say petrol prices are causing them hardship, while the issue is now ranked as more important than terrorism, education, taxes and family values.

Mr Obama leads Mr McCain by 48 to 42 per cent, similar to the margin enjoyed by John Kerry over President Bush at the same stage four years ago before his eventual defeat in the general election. The Democratic nominee currently holds double digit leads on the economy, petrol prices and energy policy.

Mr McCain, who this week declared himself the underdog in the presidential race, said his speech was the first in a series over the next fortnight setting out how he will “break our strategic dependence on oil”.

He has sought to distance himself from Mr Bush on the environment, promising to back European cap-and-trade policies for tackling global warming, while opposing oil drilling in the Arctic Wildlife Refuge and Florida’s Everglades.

“Energy conservation is no longer just a moral luxury or a personal virtue,” he said but then added: “We must also address the concerns of Americans who are struggling right now to pay for gasoline, groceries, and other necessities of life.” Mr McCain cited forecasts that petrol could soon soar to $7 a gallon because of relentless demand from high-growth economies like India or China and called for reforms to prevent “reckless wagering” by the oil futures market.

But, when there are “21 billion barrels of proven oil reserves” which cannot be exploited because of the 1981 ban, he said that Americans had to make a choice.

“It’s safe enough these days that not even Hurricanes Katrina and Rita could cause significant spillage from the battered rigs off the coasts of New Orleans and Houston,” said Mr McCain. “Yet, for reasons that become less convincing with every rise in the price of foreign oil, the federal government discourages offshore production. I believe it is time to lift these restrictions and to put our own reserves to use.”

In contrast, Mr Obama had opposed his plan to give consumers a summer “gas tax holiday” while proposing a windfall profits levy on oil firms similar to “Jimmy Carter’s big idea” in the 1970s “and a lot of good it did us”.

Mr Obama responded that plans to allow offshore drilling were, like Mr McCain’s “gas tax gimmick”, a policy that would offer limited benefits to drivers because no oil would be produced for at least a decade and, even then, “the effect on prices would be negligible”.

He added: “His decision to completely change his position and tell a group of Houston oil executives exactly what they wanted to hear today was the same Washington politics that has prevented us from achieving energy independence for decades.”

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