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 will cast ballots 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 promises to develop renewable energy sources. 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.

"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 to the coal industry's propaganda that clean coal is the hope of the future. There's no such animal as clean coal."

At stops in Pennsylvania, West Virginia, Montana and Wyoming, the Democratic 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.

Clinton also gave a nod to King Coal when she was in Charleston.

"I've been saying all along we should have clean coal, the cleanest coal possible," she said. "If we're serious about investing in clean coal and clean energy, we can create 5 million new jobs in 10 years."

Is it what coal producers and users want to hear? "Absolutely," said Joe Lucas, a vice president for the American Coalition for Clean Coal Electricity. "None of them are saying that we aren't going to need coal."