U.S. President Barack Obama's announcement of new limits on carbon emissions from the nation's existing power plants is a few days away, but the political messaging battle over what is likely to be Obama's most significant action on climate change already is heating up.

The pending rules from the Environmental Protection Agency (EPA) are widely expected to set emission-reduction targets for existing power plants and allow states to craft their own plans for how to meet the standard. Those options could include increasing efficiency, retrofitting existing plants, and boosting the production of energy from renewables and natural gas.

The EPA released the proposed rules for future power plants last fall. But new rules for emissions from existing plants—which account for