John Podesta:

Natural gas is the cleanest fossil fuel—it produces less than half as much carbon pollution as coal. Recent technology advancements make affordable the development of unconventional natural gas resources. This creates an unprecedented opportunity to use gas as a bridge fuel to a 21st-century energy economy that relies on efficiency, renewable sources, and low-carbon fossil fuels such as natural gas.

Center For American Progress Memo, Natural Gas: A Bridge Fuel for the 21st Century –