The chances of a carbon cap-and-trade scheme being introduced in the US took a major step forward this week as yet another presidential front-runner - Hillary Clinton - backed the emission reduction model.

Clinton said it was time the US took responsibility for its reign as the world's top polluter, pledging an 80 per cent cut in greenhouse gas emissions by 2050 on 1990 levels.

"This is the biggest challenge we've faced in a generation, a challenge to our economy, our security, our health, and our planet," she said in introducing her 10-year $150bn (£72bn) eco strategy. "It is time for America to meet it."

Under Clinton's cap-and-trade plan, 100 per cent of carbon credits would be auctioned to polluting companies, with the proceeds going towards new technology investments and a program to help low-income families heat and cool their homes.

She also announced plans to increase investments in energy efficiency and cleantech R&D, set targets for a quarter of US electricity to come from renewables by 2025, and introduce legislation demanding all listed firms report on climate change risks.

Cap-and-trade schemes have been the most popular green policy choice amongst the 2008 presidential favourites, with Barack Obama, John McCain and John Edwards also endorsing the strategy.

Six out of the 17 candidates have now come out in favour of cap-and-trade - a system that allows companies to buy and sell emission permits depending on how much they have polluted in relation to a specified cap.

The announcement from Clinton further increases the likelihood that the model could get the green light when a new president takes office from 2009 - particularly given growing Senate support for the recently launched Lieberman-Warner Climate Change bill and its plans for a similar cap-and-trade scheme.

The Senate sub-committee recently passed the first vote of the bill, which would legislate for a cap-and-trade system by 2012 and aim to cut harmful greenhouse gas emissions by 15 per cent before 2020.

With the environment becoming a central feature of the presidential race other leading candidates such as former New York mayor Rudy Giuliani, Mitt Romney and Fred Thompson have also endorsed green initiatives, but not specifically cap-and-trade.