Washington ? Democrats Barack Obama and Hillary Clinton are walking a delicate line as they promise to aggressively tackle global warming while trying to assure voters that they continue to believe in the future of coal.

In states like Pennsylvania, where voters cast ballots this Tuesday, and in West Virginia, Kentucky, Indiana and Montana – upcoming primary states – coal sways voters.

While increased mechanization has produced a dramatic decline in coal industry employment, the numbers remain substantial. There are 47,000 coal workers in Pennsylvania and West Virginia and 21,000 in Kentucky, according to the National Mining Association. The three states are the country’s biggest coal producers after Wyoming.

Both Obama and Clinton have rallied environmentalists with their promises to develop windmills, solar power and other renewable energy sources and order mandatory reductions in greenhouse gases from power plants to counter global warming.

It’s an energy policy that would seem to target coal, which produces half the country’s electricity but also nearly 2 billion tons of carbon dioxide, the leading greenhouse gas, each year.

Instead, “clean coal” has become the mantra of both candidates. Some environmentalists are not too happy with that.

“They keep using the term ‘clean coal.’ That’s really an oxymoron,” snaps Brent Blackwelder, president of the environmental group Friends of the Earth. “They absolutely are pandering the coal industry’s propaganda that clean coal is the hope of the future. There’s no such animal as clean coal.”

Not all environmentalists are as critical, acknowledging that coal will remain an integral part of the country’s energy picture. The two Democratic presidential aspirants’ support for coal is outweighed by their strong push for renewable fuels and – unlike President Bush – their call for mandatory, economy-wide action on climate change.

“How they finesse things on the margin is up to them,” said Cathy Duvall, the Sierra Club’s national political director, as long as they also “talk about moving away from conventional coal … and putting money into and investing in a renewable energy economy that will provide jobs.”

Obama, by representing Illinois, a top 10 coal producing state, has a little more experience at it than Clinton.

At stops in Pennsylvania, West Virginia, Montana and Wyoming, the rivals have been careful to tell voters they don’t want coal to disappear. Frequently they couch it in terms of clean, green energy development and jobs.

“We could invest in renewable sources of energy and in clean coal technology and create up to 5 million new green jobs in the bargain, including new clean coal jobs,” Obama declared at a stop in Charleston, W.Va.