The U.S. clean energy market is \"under siege\" at the federal and state levels and by \"powerful, free-spending\" opponents, the Sierra Club said in a study. In a report issued Thursday -- \"Clean Energy Under Siege: Following the Money Trail Behind the Attack on Renewable Energy\" -- the longtime environmental organization said wind energy production has doubled since 2009 and the wind industry is \"on track to produce 20 percent of America\'s electricity by 2030.\" The report said costs of generating solar energy are coming down and sales of electric cars have grown 164 percent since June 2011. \"That level of growth and success has made renewable energy more of a force to be reckoned with in energy markets,\" the report said. \"It has also drawn competitive attacks from oil, coal, and gas interests.\" The report said renewable energy is favored by large majorities of the public but \"the political spending power of the traditional energy industries is unrivaled.\" \"Clean energy is under siege at the congressional level,\" the report said, characterizing congressional investigations into a government-backed loan to the solar panel maker Solyndra as political attacks. The report said clean energy is under siege at the state level through the efforts of the American Legislative Exchange Council on behalf of oil, coal and gas industry interests, and \"well-funded fossil-fuel advocacy groups [that] masquerade as think tanks.\" \"Clean energy is under siege by some of the most powerful, free-spending entities in the nation,\" the report said. Citing reports by the campaign finance tracking group Open Secrets, the Sierra Club said oil and gas spent more than $146 million on lobbying in 2011, and \"campaign expenditures by Koch family entities Koch Industries and Oxbow Corp. place them in two of the top three campaign spending slots for 2011-2012.\" The report said oil and gas industry interests contributed to the campaigns of 88 percent of members of the U.S. House of Representatives, and 89 of 100 U.S. senators, during the 2010 election cycle -- with Republicans receiving 86 percent of all oil and gas donations.