At the United Nations climate change conference in Copenhagen in 2009 and Cancun in 2010, the United States joined other developed countries in pledging to mobilize $100 billion in public and private sector funding to help developing countries reduce greenhouse gas emissions and adapt to a warmer world. With a challenging US fiscal outlook and the failure of cap-and-trade legislation in the US Congress, America's ability to meet this pledge is increasingly in doubt. This paper identifies, quantifies, and assesses the politics of a range of potential US sources of climate finance. It finds that raising new public funds for climate finance will be extremely challenging in the current fiscal environment and that many of the politically attractive alternatives are not realistically available absent a domestic cap-and-trade program or other regime for pricing carbon. Washington's best hope is to use limited public funds to leverage private sector investment through bilateral credit agencies and multilateral development banks.