"We need some big investments right now in figuring out how to capture and store carbon dioxide from coal," Clinton told a rally in the rural town of Clear Fork on Monday.

To get there, she took a windy road through the Appalachian Mountains that passed at least four big coal mines cut into the mountainside.

Not to be outdone, Obama's campaign has distributed flyers in Kentucky stating that "Barack Obama believes in clean Kentucky coal." The flyers show a picture of giant barges carrying coal down the Ohio River.

Coal-fired power plants generate about half of US electricity supplies, and account for about 40 percent of US greenhouse gas emissions -- the biggest single industrial source.

Clinton has a plan to require US industry to cut carbon dioxide emissions by 80 percent below 1990 levels by 2050, but she hasn't brought that up in numerous appearances in West Virginia and Kentucky in recent days.

But America has 250 years worth of coal, and will likely remain the backbone of its power generation system for decades. "I know how important coal is to West Virginia," Clinton said last week in the state's capitol rotunda in Charleston. "Coal is not going anywhere for the foreseeable future."

Candidates' support for clean coal indicates a tension between their need to bring along delegate-rich coal states like Pennsylvania and Illinois and their global warming platforms.

"There is no such animal as clean coal," said Brent Blackwelder, president of the environmental group Friends of the Earth. "We shouldn't be placing our bets on coal to bail us out. We need to be looking at getting rid of coal plants."

Among Eastern US states, West Virginia and Kentucky lead the pack in coal production and employ about half of US coal industry workers -- about 39,000 people.

Both candidates support legislation that could be debated by the Senate this summer that would require US industry to cut greenhouse gas emissions by more than 70 percent by 2050.

Coal states don't hold the same clout as Farm Belt states who control about a quarter of US Electoral College votes and have pushed for higher government mandates to boost US consumption of ethanol -- made mostly from corn.

But "Big Coal" states are not to be ignored on the electoral map. And as the Democratic presidential process comes down to the wire, coal plays prominently in three of the six remaining primaries including Montana on June 3.

Coal industry officials said US electric utilities are willing to embrace carbon-reduction strategies but cannot simply shut down coal-fired plants without a massive increase in electricity prices.

"The US doesn't just have an environmental problem -- it has an energy supply problem," said Luke Popovich, spokesman for the National Mining Association. "We simply cannot zero out coal use because it is not pristine."

Not all environmental groups take such a hard line on the clean coal, pointing out that it's only natural for politicians to craft their message to their audience.

"The candidates appear to be following a tried and true tradition which is telling the audience what they want to hear," said Frank O'Donnell, president of Clean Air Watch, a nonpartisan environmental group. "It's politics as usual."

Even Al Gore, who has become a spokesman for the dangers of climate change, steered clear of talking about global warming when he campaigned in West Virginia ahead of the 2000 presidential elections, O'Donnell said.