Regardless of the future pace of ongoing reductions in U.S. carbon dioxide emissions, global carbon dioxide emissions will continue to rise. Even if the United States committed economic suicide by imposing all or most of the carbon dioxide restrictions advocated by global warming activists, the ensuing U.S. carbon dioxide reductions would amount to merely a drop in the bucket compared to the flood of emissions increases by the world as a whole and by developing nations such as China and India in particular.
There have been periods in the not-too-far past (geologically speaking) where CO2 levels have been 30 times higher than today. We are actually getting near the point where many plants will have difficulty growing (lower than 200 PPM will kill many plants) so I say CO2 is hardly our biggest concern. Much more troubling are the verifiably poisonous chemicals that industry emits. It's hardly profitable to focus on those though, so a relatively benign gas takes the brunt of the heat.
The focus on CO2 is entirely political.