Barack Obama and John McCain reiterated their support for measures designed to cut carbon emissions and enhance US energy security in last night's second presidential debate, but clashed over the details of their energy policies.

Asked to rank energy, healthcare and social security as priorities, Obama listed energy as the top priority, followed by healthcare and entitlement costs.

In contrast, McCain argued that all three could be addressed at once.

But Obama admitted the US would have to make choices over competing issues, arguing that, "we're going to have to prioritise just like a family has to prioritise."

Reiterating his plan to slash US carbon emissions and deliver energy independence from the Middle East, Obama again outlined his intention to invest $15bn (£8.6bn) a year over 10 years in bolstering renewable energy capacity.

"Our goal should be, in 10 year's time, we are free of dependence on Middle Eastern oil," he said. "And we can do it. Now, when JFK said we're going to the moon in 10 years, nobody was sure how to do it, but we understood that, if the American people make a decision to do something, it gets done."

McCain similarly committed himself to delivering energy independence, stressing the importance of nuclear power, alongside renewables and clean coal technology.

"We can work on nuclear power plants. Build a whole bunch of them, create millions of new jobs," he said. "We have to have all the above, alternative fuels, wind, tide, solar, natural gas, clean coal technology."

In an ill-tempered exchange, he also accused Obama of undermining his green credentials by backing an energy bill in 2005 that delivered tax breaks for oil companies. Controversially referring to Obama as "that one" he said Obama had supported "an energy bill on the floor of the Senate loaded down with goodies, billions for the oil companies... sponsored by Bush and Cheney".

However, while Obama did vote in favour of the Energy Policy Act of 2005, the tax cuts were offset by tax increases on oil and gas companies that delivered a net increase in their tax burden.

The debate came on the same day as new proposals for a nationwide carbon emissions cap-and-trade scheme were debuted in the House of Representatives.

The proposals, which further increase the likelihood of a US-wide cap-and-trade scheme being adopted, would cut emissions from power plants, transportation and factories by 80 per cent by 2050.

In a letter explaining the 461-page draft, House Energy and Commerce Committee chairman John Dingell and energy and air quality sub-committee chairman Rick Boucher wrote that regulation of carbon emissions was now inevitable. "Politically, scientifically, legally and morally, the question has been settled: regulation of greenhouse gases in the United States is coming," they said.

The House proposals fall between those put forward by Obama and McCain, both of whom have outlined plans for a US-wide cap-and-trade scheme.

Obama's plan would require an 83 per cent cut in emissions by mid-century, while McCain's would demand cuts of 66 per cent over the same period.

Environmentalists welcomed the move to introduce legally binding emission caps, but criticised the proposals for not requiring the cuts to be delivered as quickly as would have been delivered under the Senate's Lieberman-Warner bill, which was blocked earlier this year.

The bill was also criticised for including measures that would ban states from developing their own rules governing emissions from cars. A group of states led by California is currently suing the Environmental Protection Agency over its refusal to let them introduce vehicle emission standards that are tougher than those from the federal government.

In related news, Canadian prime minister Stephen Harper launched his Conservative Party's election campaign with a pledge to work with the US and Mexico to introduce a cap-and-trade scheme from 2015 that would operate across the entire NAFTA trade bloc.