Other major politicians, including President Bush, Barrack Obama, and Hillary Clinton, have supported clean coal as well.
The idea is pretty simple: coal is a very dirty substance, but it is also very cheap to use. And we have a LOT of it, especially in the United States. If we can find someway to reduce the pollution coal generates, we have ourselves an energy resource that will take care of us for centuries without damaging the environment.
Problem: Clean Coal prolly ain't so clean.
First, thinking that we can eliminate all the negative environmental consequences just by focusing on the "burning" part is ill-founded. Coal mining itself is considered to be highly damaging to the environment. It leaves massive scars on the earth where the mining took place, and can lead to dangerous run-offs in local ground water. Yikes! Even burning coal in some super ultra-clean fashion will not eliminate THOSE effects, and damaging the ground like that might not be such a great idea in the long-run (we might want to live in Wyoming one day, after all).
Also, even with all of our advanced technologies we have these days (including advanced scrubbing technology and burning low sulfur coal), most urban areas are STILL in violation of the Clean Air standards. I don't know what kind of technologies we can produce in the next 5 years that we hacen't already developed in the last 30 that is going to magically make coal a super-awesome source of energy.
Finally, the biggest issue facing coal plants now is carbon dioxide emissions, which are a leading cause of climate change in the world. There are currently no means for dealing with the carbon emissions from coal plants, and the most likely method (turning CO2 into liquid and shoving it back into the ground) could cost a LOT a money to implement. If it even works, that is: we're still in the experimental stage on that.
That's not to say clean coal has absolutely no role to play in our electricity policy. It's just that it is unlikely to become a bedrock for any policy that addresses environmental concerns. Many areas of the nation will still use coal energy, such as cities that require intensive amounts or areas of the country that are resource-poor in renewables.
And hence, I don't see what the fuss regarding clean coal is all about.