The two most effective ways of reducing global warming pollution — taxing it or regulating it — are politically toxic in a year when economic problems are paramount. After a bill died in the Senate in 2010, Mr. Obama abandoned his support for cap and trade, a market-based method to limit greenhouse gas emissions, and he has given little hint of what regulatory policies he intends to pursue if he wins a second term. Aides said that he would not propose a carbon tax or other energy tax, but that he would consider supporting one as part of a larger budget and spending deal.

As governor of Massachusetts, Mr. Romney considered joining a regional cap-and-trade system, then abandoned it because of uncertainty over costs. He has opposed Mr. Obama’s steps to regulate emissions from power plants and vehicles. He has said he would reverse Mr. Obama’s air quality regulations and would renegotiate the auto efficiency standard of 54.5 miles per gallon by 2025 that automakers agreed to this year.

The struggling economy has made it difficult for emerging clean energy companies to get the capital they need to reach commercial scale and compete with producers of traditional energy sources. Government programs to provide that seed money are highly controversial, as the fight over tax breaks for wind power companies and the recent failures of the solar panel maker Solyndra and the advanced battery manufacturer A123 Systems showed.

The Obama administration provided $90 billion in new financing from the 2009 stimulus for clean energy projects, but most of that money is gone.

Though there is little doubt that the burning of fossil fuels and deforestation have altered the earth’s climate, some uncertainty remains about whether and when such changes will become unmanageable. Huge technological challenges persist in transforming the energy generation system. Both Mr. Obama and Mr. Romney refer to “clean coal,” shorthand for capturing the carbon dioxide emissions from coal-burning power plants, but the technology is still in its infancy.

International efforts to address climate change, which showed great promise when Mr. Obama took office, have sputtered in recent years because of fears that limiting carbon emissions means limiting economic growth. There is also considerable resistance to any plan that would require the United States and other wealthy countries to take stronger measures than those demanded of China, India and other fast-growing economies that are responsible for the bulk of the growth in global emissions.

Mr. Romney’s chief domestic policy adviser, Oren Cass, said the nation should not take unilateral steps. “What it is going to do is hurt our economy very seriously, and it’s going to drive a lot of industrial activity from the United States to countries that are, frankly, much less efficient in their use of energy,” he said at an energy debate at the Massachusetts Institute of Technology this month.