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Where they stand:
In 2009, President Barack Obama proposed a bill that would have capped power plant carbon dioxide emissions and allowed trading of credits for the right to emit greenhouse gases, but the measure died in Congress. An international treaty effort failed. Obama since has taken a different approach, treating carbon dioxide as a pollutant under the law. He doubled auto fuel economy standards, which will increase the cost of cars but save drivers money at the pump. He's put billions of stimulus dollars into cleaner energy.
Mitt Romney's view of climate change has varied. In his book "No Apology," he wrote, "I believe that climate change is occurring" and "human activity is a contributing factor." But on the campaign trail last year he said, "We don't know what's causing climate change on this planet." He has criticized Obama's treatment of coal power plants and opposes treating carbon dioxide as a pollutant and the capping of carbon dioxide emissions, but favors spending money on clean technology. Romney says some actions to curb emissions could hurt an already struggling economy.