That would be natural gas. Because of technological change in natural gas exploration, our rates are falling, on net (offsetting increases owing to the REPS law):

Progress net rate change

Also falling: carbon dioxide emissions, as the federal Energy Information Administration has shown is mostly because of change to natural gas in electricity generation:

carbon emissions down by fracking EIA

But for some reason, a lobby effort styling itself “Conservatives for Clean Energy” wants to make people sound like they’d oppose it. Why is that?