The faltering economy and rising fuel prices are big issues for US voters
Democratic presidential hopeful Barack Obama has called for the US to dip into its strategic oil reserves to lower fuel prices in the short-term.
Senator Obama's call, a reversal of his earlier stance, came as he unveiled an energy plan designed, he said, to reduce US dependence on foreign oil.
Both Mr Obama and his Republican rival, John McCain, are focusing on ways of tackling the high cost of energy.
It is a key issue as campaigning for November's election intensifies.
Mr Obama, speaking on Monday as he celebrated his 47th birthday, put forward plans to reduce US dependence on foreign oil in 10 years.
"Breaking our oil addiction is one of the greatest challenges our generation will ever face," Mr Obama said as he visited Michigan, home to the ailing US car industry and set to be a key battleground state in the election.
"It will take nothing less than a complete transformation of our economy."
His most immediate proposal was to release 70 million barrels of crude oil held in federal stockpiles as a means of lowering petrol prices, with stocks to be replaced at a later date.
Tapping the strategic oil reserve is not a substitute for a real plan to increase supply through additional drilling and nuclear power
Tucker Bounds McCain campaign spokesman
Mr Obama had previously advocated keeping the reserve intact in case of emergency, but his spokeswoman Heather Zichal said he had reconsidered.
"He recognises that Americans are suffering," she said.
Among the other proposals in Mr Obama's speech were:
a renewed call for a $1,000 (£500) energy rebate for low and middle-income families, paid for by a windfall tax on oil companies
five million new jobs to be created by investing $150bn over the next 10 years to help private efforts on clean energy
the US to generate 10% of its electricity from renewable sources by 2012, 25% by 2025
one million fuel-efficient hybrid cars to be put on the road by 2015
The focus on energy by the two presidential hopefuls shows the increasing importance of the issue to voters.
Barack Obama outlines his plan to end US reliance on imported oil
Whichever contender captures the American mood on this is likely to capture the White House, says the BBC's Kevin Connolly in Washington.
Last week, Mr Obama modified his opposition to a federal ban on drilling off the US coast for oil, saying he could support limited and environmentally-sound drilling as part of a compromise energy package.
Senator McCain, who has changed his previous opposition to offshore drilling, now argues that it is necessary.
"Anybody who says that we can achieve energy independence without using and increasing these existing energy resources either doesn't have the experience to understand the challenge that we face or isn't giving the American people some straight talk," Mr McCain said during campaigning in Pennsylvania.
As he detailed his energy plans, Mr Obama also rolled out a new TV advert that suggested Mr McCain was under the sway of big oil firms.
John McCain now believes drilling off the US coastline is needed
The ad shows Mr McCain with President George W Bush, as a narrator says: "After one president in the pocket of big oil, we can't afford another."
In response, McCain campaign spokesman Tucker Bounds said the advert failed to mention that Mr Obama had voted in favour of a 2005 bill giving tax breaks to energy producers, a bill Mr McCain had opposed.
"Barack Obama's latest negative attack ad shows his celebrity is matched only by his hypocrisy," said Mr Bounds in a statement.
Mr Bounds also attacked Mr Obama's proposal to open up the oil reserves.
"Tapping the strategic oil reserve is not a substitute for a real plan to increase supply through additional drilling and nuclear power," he said.
"The last release of oil from the strategic reserve came in response to Hurricane Katrina, but the only crisis that has developed since Barack Obama last rejected the idea two months ago is a slide in his poll numbers."
The trading of arguments comes as opinion polls suggest neither man has a clear lead with 91 days to go until the 4 November election.
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