A measure to require that utilities produce power with a certain percentage of renewable resources is losing steam this year, its supporters say. But they are optimistic about its prospects next year with a new Congress and president.

“We’ve tried several times this Congress and we just can’t quite get it over the finish line,” said Bill Wicker, a spokesman for Jeff Bingaman, D-N.M., chairman of the Senate Energy and Natural Resources Committee.

Bingaman is a leading advocate of a “renewable portfolio standard,” which would require electric utilities to generate a certain percentage of their power from renewable resources such as wind, solar energy and geothermal sources. Legislative proposals have ranged from 15 percent to 25 percent.

“We’re going to bring it back as a red-hot, No. 1 issue” in the next Congress, Wicker said.

Their hope is that Democrats widen their margins of control in the House and Senate and capture the White House. But even that does not guarantee enactment of a renewable standard.

Experts say that in a sluggish economy, lawmakers and a new president may have to choose between a broad measure to cap greenhouse gas emissions or a narrower bill that applies to just one sector, like the renewable portfolio standard.

Although 25 states and the District of Columbia now have state renewable-power mandates, less than 5 percent of the nation’s electricity is generated from renewable sources.

The proposed national mandate would require a threefold to fivefold increase over current levels of renewable power and would fundamentally reshape how electric utilities do business.

Similar measures have passed the Senate three times in recent years and passed the House last fall, but regional differences derailed efforts to include a renewable-power mandate in energy legislation (PL 110-140) enacted in December 2007.

Supporters say a national renewable mandate would be a major step toward addressing climate change and dependence on fossil fuels. Critics fear that it would raise electric bills for consumers in regions with limited solar and wind resources, such as the Southeast.

As Texas governor, President Bush signed a statewide renewable-power mandate, but he has long opposed imposing a nationwide standard.

Both Sens. Hillary Rodham Clinton of New York and Barack Obama of Illinois, the Democratic presidential rivals, support requiring the nation’s utilities to produce as much as 25 percent of electricity from renewable sources by 2025.

GOP presidential hopeful Sen. John McCain of Arizona voted against establishing a national renewable portfolio standard in 2005 but has been a leading supporter of climate-change legislation that would mandate caps on carbon emissions and promote renewable-energy sources over fossil fuels.

The McCain campaign’s energy adviser, former CIA chief James Woolsey, is a vociferous proponent of renewable electricity, especially solar.

There may be limits, however, on how much even a supportive Congress and sympathetic figure in the White House can accomplish next year.

“Will the new president be willing to pass both a [renewables mandate] and a cap-and-trade system, or is it going to be a trade-off?” asked Paul Bledsoe, a spokesman for the bipartisan National Commission on Energy Policy, which advises Congress on energy matters. “If industry has to accept regulation of [carbon dioxide], is it going to accept additional regulation?”

Still, Bledsoe said the prospects for doing something to encourage cleaner power appear certain to improve after November.

“After the elections, there’s going to be a broader attempt to promote renewable energy,” he said.