The London-based investment company has $3.3 billion under management in its renewable energy funds, and Locklin said it would consider investing in wind-farm MLPs if it could.

Wind and solar projects currently qualify for tax grants from the U.S. Treasury of as much as 30 percent of their construction costs. That program is set to expire at the end of this year. Underlying tax credits expire for wind in 2012 and for solar in 2016.

The credits helped make wind farms the second-fastest growing source of new electricity after natural gas, and trade groups led by the American Wind Energy Association and the American Council on Renewable Energy are lobbying to make master limited partnerships a long-term replacement to help the industry compete with fossil fuels.

'Cheap Financing'

"I sit here and smile every time I hear the oil and gas industry complain about losing tax breaks that they've had for a 100 years," Denise Bode, chief executive officer of the American Wind Energy Association in Washington, said in an interview. "They got their start with cheap financing from MLPs and we want that, too."

The wind group, along with companies such as NextEra Energy Inc., Shell Wind Energy Inc. and General Electric Co., are in the Renewables for Publicly Traded Partnerships Group, according to a lobbying registration form filed July 15. The group hired Capitol Counsel LLC, which includes former Representative Jim McCrery of Louisiana, top Republican on the House Ways and Means Committee in 2007 and 2008.

'Tough Climate'

The coalition is making the request in a "tough climate" as Congress debates deficit reduction, McCrery said. The group, which also includes a unit of Vestas Wind Systems A/S, the world's biggest turbine maker, and BrightSource Energy Inc., which builds solar-thermal plants, hopes bipartisan support for renewable energy will help advance the request.

"We've got this problem where investors, companies can't use tax credits that were designed to make the investments a workable, financial deal," he said. "We don't really care how we solve the problem, but if we don't solve the problem, we're going to have fewer and fewer dollars invested in renewables."

Forgoing corporate taxes on MLPs may cost the Treasury about $2.9 billion in revenue through 2014, according to the National Association of Publicly Traded Partnerships, citing a December report from the Joint Committee on Taxation. The Alexandria, Virginia-based trade group represents MLPs.

About 75 percent of MLPs now are in the energy industry. The largest pipeline company formed under the structure is Enterprise Products Partners LP, followed by Kinder Morgan Energy Partners. Tesoro Corp. the largest independent refiner in the U.S., in April separated its pipeline assets from its refining business to form Tesoro Logistics LP and avoid corporate income tax.

Indexes Gain

The Alerian MLP Index of 50 energy MLP's has gained 15 percent in the past year, compared with a 0.2 percent decline in the Wilderhill New Energy Global Innovation Index of 100 clean energy companies over the same period. The S&P 500 index has gained 21 percent.