In a rare show of bipartisanship, a group of Democratic and Republican senators introduced legislation on Tuesday that would require utilities nationwide to generate at least 15 percent of their electricity from renewable sources like wind, solar and biomass by 2021.

The bill was introduced by Jeff Bingaman, Democrat of New Mexico, and Sam Brownback, Republican of Kansas.

“I think that the votes are present in the Senate to pass a renewable electricity standard,” Mr. Bingaman said in a statement. “I think that they are present in the House. I think that we need to get on with figuring out what we can pass and move forward.”

The bill counts two other Republicans as co-sponsors: Susan Collins of Maine and John Ensign of Nevada. The bill may need several more Republicans on board to clear the 60- vote hurdle to end debate in the Senate, however, as some Southeastern and Midwestern Democrats may be likely to oppose the measure.

Senator Mary Landrieu, Democrat of Louisiana, said in an interview with the news service Energy and Environment Daily that she could not support a renewable electricity standard unless the Obama administration’s moratorium on deepwater drilling in the Gulf of Mexico, set to expire on Nov. 30, was lifted. “It just has to happen,” Ms. Landrieu said.

A standalone renewable electricity standard would not have her support, she said. “If that’s all it is, it’s not even worth me talking about it.”

The Democrats Blanche Lincoln of Arkansas, Ben Nelson of Nebraska and Evan Bayh of Indiana are also considered potential “no” votes on the measure. Ms. Lincoln and Mr. Nelson have expressed doubts about such standards in the past, and Mr. Bayh voted against a renewable electricity standard in a 2009 committee vote.

A nearly identical electricity mandate was drafted by Mr. Bingaman and passed by the Senate Energy and Environment Committee in 2009, but faced opposition from renewable energy groups and Democrats inclined toward a more ambitious target. But with cap-and-trade legislation all but dead in the Senate, and the prospect looming that no clean energy or climate legislation at all will be passed this year, some of those who previously opposed the measure have hopped on board.

“The R.E.S. passed by the Senate Energy Committee in 2009 is not as strong as it should be, but it would establish a first-ever national framework for increasing the use of renewable electricity,” said Marchant Wentworth of the Union of Concerned Scientists, which previously opposed the 15 percent mandate as far too weak. “That is a crucial step toward a lower-carbon economy, and we must take it now.”