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Senator John Kerry (D-MA), right, and Senator Joe Lieberman (I-CT), left, speak at a press conference on climate change. - 

Nancy Marshall Genzer: The Senate bill being introduced today is expected to set a cap on domestic greenhouse gas emissions. But Senate leaders are leaning toward a smaller energy bill. It requires utilities to get 15 percent of their power from renewable sources like solar and wind power, by 2021. It would be the first ever federal mandate on renewable energy.

Frank O'Donnel: A number of states have already done this. But there's been no federal requirement. So it would be a modest step.

That's Clean Air Watch president Frank O'Donnell. If you're thinking he doesn't sound too enthused, you're right. He says the legislation's definition of renewable energy is too broad. It includes things like nuclear power.

Marchant Wentworth is a lobbyist with the Union of Concerned Scientists. He says the oil spill in the Gulf gives the bigger energy bill a boost.

Marchant Wentworth: Actually, this focuses the American people on the need for progress on moving away from carbon fuels.

The House passed a sweeping energy bill almost a year ago.

In Washington, I'm Nancy Marshall Genzer for Marketplace.