Ohio, USA (AFP) — White House hopeful Mitt Romney returned yesterday to the battleground state of Ohio, pressing a comprehensive energy policy and blasting President Barack Obama for waging "a war on coal."

Fresh from an intense weekend of campaigning with his newly-minted running mate, congressman Paul Ryan, Romney is wrapping up a swing-state bus tour in this Midwestern state, making an appearance at a coal mine in the village of Beallsville.

Ohio is a major coal producer, and much of its energy comes from the fossil fuel that the presumptive Republican nominee has touted in several campaign stops this year as he rallies voters around his pledge to expand domestic energy production.

Both candidates say they support boosting energy production such as natural gas that would help the US become energy independent.

The Obama administration says clean coal is part of the president's "all of the above" energy strategy, and it has funded research and development in low-emissions coal.

But Republicans are up in arms about Environmental Protection Agency regulations approved last year that limit emissions of toxic substances by coal-burning power plants, saying the rules are overly burdensome and have forced dozens of coal plants to close.

Romney's campaign has called the policies "disastrous."

"In Ohio and around the nation, the president's energy policies have resulted in fewer jobs and higher costs at home," Romney spokeswoman Andrea Saul said in a statement.

"Across the nation, the Obama administration has waged a war on coal."

As Romney tours Ohio, Obama has been on a three-day bus tour rolling across Iowa, where some 20 per cent of the state's electricity is generated by wind power.

Obama has hammered Romney on his opposition to the extension of a wind energy tax credit designed to encourage green technology which is due to expire at the end of the year.

Romney's campaign has said he would let the credit expire because he thinks government should not be in the business of supporting one industry over another.