Despite whittling down a proposed federal renewable-energy mandate to a fraction of its previous strength, Sen. Jeff Bingaman, D-N.M., still faces a tough sell to gain enough support for passage.

Narrow votes on amendments to the proposal in the Senate Energy and Natural Resources panel Thursday outlined the hurdles the legislation faces, first in a committee vote and then to the chamber floor as part of a broad energy bill later this summer.

Bingaman's proposal requires utilities to provide an increasing percentage of power from sources such as wind, sunshine and geothermal energy. The mandate starts at 3% in the first 2011-2013 compliance period and rises to 15% by 2021. To help companies meet the standard, they're allowed to buy renewable-energy credits or pay a 2.1 cents-a-kilowatt-hour fee. In addition, states can allow up to a quarter of the requirement can be met through energy- efficiency gains.

Some Democrats want to strengthen the bill, saying that it has been watered down too much and offering amendments to raise the target to 25% by 2025 or strip out the efficiency-offsets provision. Those amendments were withdrawn but will likely be offered again when the broader energy bill comes to the Senate floor.

"It's simply not bold enough," said Sen. Robert Menendez, D-N.J. Given inclusion of efficiency credits, the fact that new nuclear power won't be accounted in the mandate, and small utilities are exempt, Menendez said the actual renewable mandate will translate into a 9% requirement in 2021.

Bingaman, like some Democrats, would like much higher targets. The committee chairman, having weathered a raft of unsuccessful attempts to pass a renewable mandate in the Senate in previous years, calculated his compromise as the path to passage.

Although Bingaman may want to push for a 25% standard, "he just recognizes the balance that has to be maintained if we're really going to move anywhere," said Sen. Sam Brownback, R-Kan. More stringent standards would translate into higher utilities bills, and, "boy, if that starts to happen to too many people, I think you lose the whole thing."

Many Republicans and some Democratic Senators - particularly from southeastern states that don't have the same natural renewable energy endowments others have - want to include nuclear power in the mandate or boost the amount credit companies can get for energy-efficiency gains. Although both options would lead to a cut in greenhouse-gas emissions and address energy-security concerns, critics say they would dilute incentives for fueling a "green economy."

Signaling the close vote expected on the floor, the amendments to include nuclear power and boost efficiency failed in the committee by 12-11 votes.

The GOP is also pressing for expanded oil and gas access in federal waters, beyond simply a new inventory of resources that Bingaman has currently offered.

The renewable mandate is part of a bigger package, said ranking member Sen. Lisa Murkowski, R-Alaska. "You have to remember that this is not just in a vacuum," she told reporters when asked if she would support the chairman's compromise.

"We need a meaningful domestic production piece," Murkowski said.

Robert Dillon, spokesman for the panel's minority, said, "Some members said if they could get some real production out of this, they could hold their noses and vote for the renewable mandate."

The panel will next week consider the oil and gas segment of the energy bill.

The committee did approve a Republican amendment that would allow companies to seek temporary relief from the mandate if bottlenecks in transmission prevented renewable-production growth. Energy analysts warn that a lack of grid capacity represents one of the biggest barriers to renewable-energy growth in the years ahead.

Murkowski said there was also potential for wide support for an amendment that would allow 100% of the target in the first few years of the program to be met through efficiency gains.

Southern utility companies, including Duke Energy Corp. (DUK) and Southern Co. (SO), have lobbied against a federal renewable portfolio standard, saying it would put an unfair burden on states and consumers.

Wind turbine manufacturers such as GE Energy, a unit of the General Electric Co. (GE), India's Suzlon Energy (532667.BY) and Denmark's Vestas Wind Systems (VWS.OS), as well as solar firms such as Norway's Renewable Energy Corp. ASA (REC.OS), and U.S.-headquartered First Solar Inc. (FSLR) and Evergreen Solar Inc. (ESLR) would benefit under a renewable portfolio standard.

Bingaman said he hoped to pass the entire bill out of committee next week.

-By Ian Talley, Dow Jones Newswires; (202) 862 9285; ian.talley@dowjones.com